503 lines
22 KiB
Python
503 lines
22 KiB
Python
import copy
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import logging
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from sage.all import sage_eval, PolynomialRing, QQ, SR, log, matrix, prod, var
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load("sage/util.py")
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# Logger
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logger = logging.getLogger(__name__)
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logging.basicConfig(level=logging.DEBUG)
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class PFOperator:
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"""
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A class representing a Picard-Fuchs operator in a single variable z.
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Independent of the given order, the variables are assumed to be left of
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the derivatives.
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The variables extra_locals are needed for symbolic coefficients used for
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operator Ansätze.
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"""
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def _operator_from_string(self, operator_string: str, extra_locals: dict = None):
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z_names = ['z%d' % i for i in range(self.no_variables)]
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theta_names = ['theta%d' % i for i in range(self.no_variables)]
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base_ring = SR if extra_locals else QQ
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self.ring = PolynomialRing(base_ring, z_names + theta_names)
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self.z_gens = self.ring.gens()[:self.no_variables]
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self.theta_gens = self.ring.gens()[self.no_variables:]
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parse_locals = dict(self.ring.gens_dict())
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if extra_locals:
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parse_locals.update(extra_locals)
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try:
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operator = self.ring(sage_eval(operator_string, locals=parse_locals))
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return operator
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except Exception as e:
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raise ValueError("Invalid operator string: %s" % e)
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def __init__(self, operator_string: str, no_variables: int = 1, extra_locals: dict = None):
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self.no_variables = no_variables
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self.operator_string = operator_string
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self.operator = self._operator_from_string(operator_string, extra_locals=extra_locals)
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logger.info("Initialised PFOperator: %s", self.operator)
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def simplify(self) -> "PFOperator":
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"""
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Groups the operator's terms by z-monomial and factorises the theta-polynomial
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multiplying each z-monomial, e.g. turning a computed quintic operator into the
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well-known theta0^4 - 5*z0*(5*theta0 + 1)*(5*theta0 + 2)*(5*theta0 + 3)*(5*theta0 + 4).
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"""
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z_monomial_theta_parts = {}
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for coeff, monomial in self.operator:
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exponents = monomial.exponents()[0]
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z_exponents = exponents[:self.no_variables]
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theta_exponents = exponents[self.no_variables:]
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theta_monomial = SR(1)
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for gen, exp in zip(self.theta_gens, theta_exponents):
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theta_monomial *= SR(gen) ** exp
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z_monomial_theta_parts[z_exponents] = z_monomial_theta_parts.get(z_exponents, SR(0)) + SR(coeff) * theta_monomial
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simplified_expr = SR(0)
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for z_exponents, theta_part in z_monomial_theta_parts.items():
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z_monomial = SR(1)
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for gen, exp in zip(self.z_gens, z_exponents):
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z_monomial *= SR(gen) ** exp
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simplified_expr += theta_part.factor() * z_monomial
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simplified = copy.copy(self)
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simplified.operator_string = str(simplified_expr)
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logger.debug("Simplified PFOperator to: %s", simplified.operator_string)
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return simplified
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class PFOperatorAnsatz(PFOperator):
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"""
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A class for Picard-Fuchs operator Ansätze, characterised by number of variables and their z- and theta-multi-degrees.
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"""
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def __init__(self, no_variables: int, theta_degree: int, z_degree: int):
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self.no_variables = no_variables
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self.theta_degree = theta_degree
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self.z_degree = z_degree
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z_indices = _multi_indices(self.no_variables, z_degree)
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theta_indices = _multi_indices(self.no_variables, theta_degree)
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# Every (z_index, theta_index) pair allowed by the degree bounds gets its own
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# fresh unknown, to be solved for once the Ansatz is applied to a period.
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operator_terms = [
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(var("b_" + "_".join(str(x) for x in z_index + theta_index)), (z_index, theta_index))
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for z_index in z_indices
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for theta_index in theta_indices
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]
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self.unknowns = [coeff for coeff, _ in operator_terms]
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def _monomial_factors(index, name):
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return [f"{name}{i}^{exp}" for i, exp in enumerate(index) if exp > 0]
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operator_string = " + ".join(
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"*".join([str(coeff), *_monomial_factors(z_index, "z"), *_monomial_factors(theta_index, "theta")])
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for coeff, (z_index, theta_index) in operator_terms
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)
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extra_locals = {str(coeff): coeff for coeff in self.unknowns}
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super().__init__(operator_string=operator_string, no_variables=no_variables, extra_locals=extra_locals)
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class PFIdeal:
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"""
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A class representing a Picard-Fuchs ideal, which is a collection of PFOperators.
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"""
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def __init__(self, operators: list):
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if not all(op.no_variables == operators[0].no_variables for op in operators):
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raise ValueError("All operators must have the same number of variables.")
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self.no_variables = operators[0].no_variables
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self.operators = operators
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logger.info("Initialised PFIdeal with %d operator(s).", len(self.operators))
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def add_operator(self, pf_operator: PFOperator):
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self.operators.append(pf_operator)
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logger.info("Added PFOperator to PFIdeal: %s", pf_operator.operator_string)
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def remove_operator(self, pf_operator: PFOperator):
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self.operators.remove(pf_operator)
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logger.info("Removed PFOperator from PFIdeal: %s", pf_operator.operator_string)
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def find_power_series_solution(self, indicials: list, order: int) -> list:
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"""
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Finds power series solutions (no logs) to this PFIdeal at given indicial exponents/order.
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A term c*z^p*theta^q sends a_k*z^k (true exponent k+indicials) to
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c*(k+indicials)^q*a_k*z^(k+p): it shifts index k up by p (p>=0), never down or sideways.
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E.g. z0*theta0 sends a_k*z0^k to (k+rho0)*a_k*z0^(k+1).
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This is the multivariate Frobenius method: canonical series solutions of a regular
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holonomic D-ideal via its indicial ideal. See M. Saito, B. Sturmfels, N. Takayama,
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"Gröbner Deformations of Hypergeometric Differential Equations", Algorithms and
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Computation in Mathematics vol. 6, Springer, 2000, chs. 2-3.
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"""
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if len(indicials) != self.no_variables:
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raise ValueError(
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"Indicials must have length no_variables=%d, got %d." % (self.no_variables, len(indicials))
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)
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indicials = [QQ(rho) for rho in indicials]
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operator_terms = [
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[
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(monomial.exponents()[0][:self.no_variables], monomial.exponents()[0][self.no_variables:], coeff)
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for coeff, monomial in pf_operator.operator
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]
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for pf_operator in self.operators
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]
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def eigenvalue(k, q):
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# theta_i^q_i acts on z_i^(k_i + indicials[i]) as multiplication by
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# (k_i + indicials[i])^q_i; theta^q's combined eigenvalue is the product over i.
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value = QQ(1)
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for i in range(self.no_variables):
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if q[i]:
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value *= (k[i] + indicials[i]) ** q[i]
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return value
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# Process multi-indices in order of increasing total degree: since every operator
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# monomial has p >= 0 (componentwise), the z^m coefficient of L(y) only ever
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# depends on a_k for k <= m, so by this point every k < m has already been solved.
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z_indices = sorted(_multi_indices(self.no_variables, order), key=sum)
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# solved[k] holds a_k written as a vector of coefficients over the `dimension`
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# independent solutions found so far (a basis of the solution space up to k).
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dimension = 0
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solved = {}
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for m in z_indices:
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# For each operator, "coefficient of z^m in L(y) = 0" splits into a diagonal
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# part (the p=0, theta-only monomials, whose unknown is a_m itself) plus a
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# known part contributed by already-solved a_k with k = m - p, p > 0.
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diagonals = []
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known_contributions = []
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for terms in operator_terms:
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diagonal = QQ(0)
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contribution = [QQ(0)] * dimension
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for p, q, coeff in terms:
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k = tuple(m[i] - p[i] for i in range(self.no_variables))
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if any(ki < 0 for ki in k):
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continue # this monomial would need a_k for a negative multi-index k: no such term
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ev = eigenvalue(k, q)
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if ev == 0:
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continue # theta^q kills z_i^(k_i + indicials[i]) here, so this monomial contributes nothing
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if k == m:
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diagonal += coeff * ev # p = 0: coefficient multiplying the still-unknown a_m
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else:
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k_vector = solved[k] # p > 0: a_k is already known, add its contribution
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for i in range(dimension):
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contribution[i] += coeff * ev * k_vector[i]
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diagonals.append(diagonal)
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known_contributions.append(contribution)
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# An operator with a nonzero diagonal lets us solve a_m = -(known part)/diagonal
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# directly; this is exactly the indicial equation being nonzero at m + indicials.
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active = next((r for r, d in enumerate(diagonals) if d != 0), None)
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if active is not None:
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value = [-known_contributions[active][i] / diagonals[active] for i in range(dimension)]
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# Every operator's equation at m must independently be satisfied by this
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# same a_m; disagreement means the ideal is inconsistent with these indicials.
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for r, d in enumerate(diagonals):
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if any(d * value[i] + known_contributions[r][i] != 0 for i in range(dimension)):
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raise ValueError(
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"Inconsistent Picard-Fuchs ideal or indicial exponents %s at multidegree %s."
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% (indicials, m)
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)
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solved[m] = value
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else:
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# Resonance: every operator's indicial part vanishes at m + indicials, so a_m
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# cannot be pinned down by this equation. If the already-known lower-degree
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# data still forces a nonzero constraint here, satisfying it would require a
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# log(z)-term solution, which this method (deliberately) does not compute.
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if any(x != 0 for contribution in known_contributions for x in contribution):
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raise ValueError(
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"Resonance at multidegree %s for indicials %s would require a logarithmic "
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"solution, which find_power_series_solution does not compute." % (m, indicials)
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)
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# Otherwise a_m is genuinely free: it starts a new independent solution, so
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# extend every previously solved coefficient with a 0 in this new direction.
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dimension += 1
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for v in solved.values():
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v.append(QQ(0))
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solved[m] = [QQ(0)] * (dimension - 1) + [QQ(1)]
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log_index = tuple([0] * self.no_variables)
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solutions = [
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Period(
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no_variables=self.no_variables,
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coefficients={log_index: {m: solved[m][i] for m in z_indices}},
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order=order,
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indicials=indicials,
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)
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for i in range(dimension)
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]
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logger.info(
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"Found %d power series solution(s) for indicials %s at order %d.", len(solutions), indicials, order
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)
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return solutions
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class Period:
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"""
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A class representing a period as a formal power series in z-variables and their logs.
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The coefficients are stored in a dictionary of dictionaries, where the first key is the multi-index
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of the logarithmic part and the second key is the multi-index of the z-variables.
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So, for example, the coefficient of
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log(z0)^2 * log(z1) * z0^3 * z1^2 would be stored as coefficients[(2, 1)][(3, 2)]
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"""
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def _initialise_ring(self):
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z_names = ['z%d' % i for i in range(self.no_variables)]
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log_names = ['L%d' % i for i in range(self.no_variables)]
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ring = PolynomialRing(QQ, z_names + log_names)
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z_gens = ring.gens()[:self.no_variables]
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log_gens = ring.gens()[self.no_variables:]
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return ring, z_gens, log_gens # type: (PolynomialRing, list, list)
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def _period_from_string(self, period_string: str) -> dict:
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ring, z_gens, log_gens = self._initialise_ring()
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log_of_z = dict(zip(z_gens, log_gens))
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def log(zi):
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try:
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return log_of_z[zi]
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except (KeyError, TypeError):
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raise ValueError(
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"log(...) may only be applied to one of the z-variables z0, ..., z%d"
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% (self.no_variables - 1)
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)
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parse_locals = dict(ring.gens_dict())
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parse_locals['log'] = log
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try:
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expression = ring(sage_eval(period_string, locals=parse_locals))
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except Exception as e:
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raise ValueError("Invalid period string: %s" % e)
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coefficients = {}
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for coeff, monomial in expression:
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exponents = monomial.exponents()[0]
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z_index = tuple(exponents[:self.no_variables])
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log_index = tuple(exponents[self.no_variables:])
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coefficients.setdefault(log_index, {})[z_index] = coeff
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return coefficients
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def _period_to_string(self) -> str:
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ring, z_gens, log_gens = self._initialise_ring()
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expression = ring(0)
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for log_index, z_dict in self.coefficients.items():
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for z_index, coeff in z_dict.items():
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monomial = coeff
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for i in range(self.no_variables):
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monomial *= (log_gens[i] ** log_index[i]) * (z_gens[i] ** z_index[i])
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expression += monomial
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log_substitutions = {log_gens[i]: log(SR(z_gens[i])) for i in range(self.no_variables)}
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result = SR(expression).subs(log_substitutions)
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# A nonzero indicial ρ_i means the coefficients above are for z_i^k, but the
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# actual solution is z_i^(ρ_i + k); make that explicit in the printed form.
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indicial_prefactor = prod(
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(SR(z_gens[i]) ** self.indicials[i] for i in range(self.no_variables) if self.indicials[i] != 0),
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SR(1),
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)
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if indicial_prefactor != 1:
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result *= indicial_prefactor
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return str(result)
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def _max_z_degree(self) -> int:
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# Highest total z-degree (sum of the z-multi-index) among all coefficients.
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if not self.coefficients:
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return 0
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return max(
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sum(z_index)
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for z_dict in self.coefficients.values()
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for z_index in z_dict
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)
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def _truncate_coefficients(self, order: int) -> dict:
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# Drop every (log_index, z_index) entry whose total z-degree exceeds order,
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# removing it from the dictionary rather than merely zeroing it out.
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truncated = {}
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for log_index, z_dict in self.coefficients.items():
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kept = {z_index: coeff for z_index, coeff in z_dict.items() if sum(z_index) <= order}
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if kept:
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truncated[log_index] = kept
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return truncated
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def _apply_theta(self, coefficients: dict, index: int) -> dict:
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"""
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Applies the logarithmic derivative theta_i = z_i * d/dz_i once to a coefficients dict of the same
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shape as self.coefficients. It uses the product rule
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theta_i(z^a log(z)^k) = a_i * z^a log(z)^k + k_i * z^a log(z)^(k - e_i).
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z_index entries are offsets from self.indicials: a stored z_index of a really means
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z^(a + self.indicials[index]), so theta_i's eigenvalue is a_i + self.indicials[index]
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rather than the bare a_i (self.indicials is all-zero unless explicitly given, in which
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case this reduces to the ordinary power-series rule).
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"""
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result = {}
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for log_index, z_dict in coefficients.items():
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for z_index, coeff in z_dict.items():
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a_i = z_index[index] + self.indicials[index]
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if a_i != 0:
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inner = result.setdefault(log_index, {})
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inner[z_index] = inner.get(z_index, 0) + a_i * coeff
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k_i = log_index[index]
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if k_i != 0:
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lowered_log_index = log_index[:index] + (k_i - 1,) + log_index[index + 1:]
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inner = result.setdefault(lowered_log_index, {})
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inner[z_index] = inner.get(z_index, 0) + k_i * coeff
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return result
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def apply_operator(self, pf_operator: PFOperator) -> "Period":
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"""
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Applies a PFOperator to this period and returns the result as a new Period,
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truncated to self.order. Each monomial of the operator is normalised as
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coeff * z^p * theta^q (see PFOperator's docstring), so theta^q is applied
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to the period first and the result is then multiplied by coeff * z^p.
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"""
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if pf_operator.no_variables != self.no_variables:
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raise ValueError(
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"Variable count mismatch: period has %d variable(s), operator has %d."
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% (self.no_variables, pf_operator.no_variables)
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)
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result_coefficients = {}
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for monomial_coeff, monomial in pf_operator.operator:
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exponents = monomial.exponents()[0]
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z_exponents = exponents[:self.no_variables]
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theta_exponents = exponents[self.no_variables:]
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term = self.coefficients
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for i, power in enumerate(theta_exponents):
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for _ in range(power):
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term = self._apply_theta(term, i)
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for log_index, z_dict in term.items():
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inner = result_coefficients.setdefault(log_index, {})
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for z_index, coeff in z_dict.items():
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shifted_z_index = tuple(z_index[i] + z_exponents[i] for i in range(self.no_variables))
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inner[shifted_z_index] = inner.get(shifted_z_index, 0) + monomial_coeff * coeff
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result_coefficients = {
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log_index: {z_index: c for z_index, c in z_dict.items() if c != 0}
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for log_index, z_dict in result_coefficients.items()
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}
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result_coefficients = {log_index: z_dict for log_index, z_dict in result_coefficients.items() if z_dict}
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return Period(
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no_variables=self.no_variables,
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coefficients=result_coefficients,
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order=self.order,
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indicials=self.indicials,
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)
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def find_annihilating_operators(self, z_degree: int, theta_degree: int) -> list:
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"""
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Finds operators annihilating this period among PFOperator Ansätze of the given
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z- and theta-degree.
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"""
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ansatz = PFOperatorAnsatz(self.no_variables, theta_degree, z_degree)
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unknowns = ansatz.unknowns
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equations = [
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coeff
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for z_dict in self.apply_operator(ansatz).coefficients.values()
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for coeff in z_dict.values()
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]
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coefficient_rows = [[SR(equation).coefficient(b) for b in unknowns] for equation in equations]
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kernel_basis = matrix(QQ, coefficient_rows, ncols=len(unknowns)).right_kernel().basis()
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operators = []
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for basis_vector in kernel_basis:
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||
solution = {unknowns[j]: basis_vector[j] for j in range(len(unknowns))}
|
||
solved_operator = ansatz.operator.map_coefficients(lambda c: SR(c).subs(solution))
|
||
operators.append(PFOperator(str(solved_operator), no_variables=self.no_variables))
|
||
|
||
return operators
|
||
|
||
def __init__(
|
||
self,
|
||
no_variables: int = 1,
|
||
coefficients: dict = None,
|
||
period_string: str = None,
|
||
order: int = None,
|
||
indicials: list = None,
|
||
):
|
||
self.no_variables = no_variables
|
||
# indicials[i] is the (rational) Frobenius exponent ρ_i of z_i: a stored
|
||
# z-index of a really represents z^(a + indicials[i]).
|
||
self.indicials = list(indicials) if indicials is not None else [0] * no_variables
|
||
|
||
# Use coefficients or period_string to initialize the period
|
||
if period_string is not None:
|
||
if coefficients is not None:
|
||
raise ValueError("Provide either coefficients or period_string, not both.")
|
||
self.coefficients = self._period_from_string(period_string)
|
||
self.period_string = period_string
|
||
logger.debug("Using string for initialisation.")
|
||
elif coefficients is None:
|
||
self.coefficients = {}
|
||
else:
|
||
self.coefficients = coefficients
|
||
self.period_string = self._period_to_string()
|
||
logger.debug("Using coefficients for initialisation.")
|
||
|
||
if order is None:
|
||
self.order = self._max_z_degree()
|
||
logger.debug("Order not provided, using maximum z-degree: %d", self.order)
|
||
else:
|
||
self.coefficients = self._truncate_coefficients(order)
|
||
self.order = order
|
||
self.period_string = self._period_to_string()
|
||
logger.debug("Truncated coefficients to order %d.", self.order)
|
||
|
||
logger.info("Initialised Period in %d variables at order %d.", self.no_variables, self.order)
|
||
|
||
|
||
class PeriodAnsatz(Period):
|
||
"""
|
||
A class for period Ansätze, characterised by number of variables and their z- and log-multi-degrees.
|
||
"""
|
||
|
||
def __init__(self, no_variables: int, z_degree: int, log_degree: int, indicials: list = None):
|
||
self.no_variables = no_variables
|
||
self.z_degree = z_degree
|
||
self.log_degree = log_degree
|
||
|
||
log_indices = _multi_indices(self.no_variables, log_degree)
|
||
z_indices = _multi_indices(self.no_variables, z_degree)
|
||
|
||
# Every (log_index, z_index) pair allowed by the degree bounds gets its own
|
||
# fresh unknown, to be solved for once a PFOperator is applied to the Ansatz.
|
||
coefficients = {
|
||
log_index: {
|
||
z_index: var("a_" + "_".join(str(x) for x in log_index + z_index))
|
||
for z_index in z_indices
|
||
}
|
||
for log_index in log_indices
|
||
}
|
||
|
||
super().__init__(no_variables=no_variables, coefficients=coefficients, order=z_degree, indicials=indicials)
|
||
self.expansion_coefficients = self.coefficients
|
||
logger.info(
|
||
"Initialised PeriodAnsatz with %d unknown coefficient(s).",
|
||
sum(len(z_dict) for z_dict in self.expansion_coefficients.values()),
|
||
) |