Adding test for 4.2.1

tests/test_period_computation.py

@@ -40,14 +40,6 @@ def test_apply_operator_truncates_to_order():
     assert result.order == 2
 
 
-def test_apply_operator_rejects_variable_count_mismatch():
-    period = Period(no_variables=1, coefficients={})
-    op = PFOperator("z0*theta1", no_variables=2)
-
-    with pytest.raises(ValueError):
-        period.apply_operator(op)
-
-
 def test_find_annihilating_operators_recovers_theta_squared():
     # theta0^2 annihilates log(z0): theta0(log z0) = 1, theta0^2(log z0) = theta0(1) = 0.
     # Among degree-(0, 2) Ansatze this should be the only solution, up to scaling.
@@ -134,8 +126,7 @@ def test_find_power_series_solution_handles_rational_indicial():
 
 
 def test_ideal_finds_holomorphic_solution_and_recovers_first_operator():
-    # Ideal generated by two operators (paper's z1, z2, theta1, theta2 <-> code's z0, z1,
-    # theta0, theta1):
+    # Picard--Fuchs ideal for P_{22211}[8]:
     #   M1 = theta2*(-2*theta1+2*theta2-1) + 2*(theta1-2*theta2-1)*(theta1-2*theta2)*z2
     #   M2 = theta1^2*(2*(theta1-2*theta2)*z2-theta2) - 16*(2*theta1+1)*(4*theta1+1)*(4*theta1+3)*z1*z2
     M1 = PFOperator(
@@ -171,3 +162,34 @@ def test_ideal_finds_holomorphic_solution_and_recovers_first_operator():
     )
     assert ratio != 0
     assert recovered[0].operator == ratio * M1.operator
+
+
+def test_single_operator_recovers_itself_from_its_holomorphic_period():
+    # Ensuring methods work for operator 4.2.1:
+    L = PFOperator(
+        "theta0^4 - 4*z0*(2*theta0 + 1)^2*(7*theta0^2 + 7*theta0 + 2) "
+        "- 128*z0^2*(2*theta0 + 1)^2*(2*theta0 + 3)^2",
+        no_variables=1,
+    )
+    ideal = PFIdeal([L])
+
+    solutions = ideal.find_power_series_solution(indicials=[0], order=15)
+    assert len(solutions) == 1
+
+    period = solutions[0]
+    assert period.apply_operator(L).coefficients == {}
+    assert period.coefficients[(0,)][(0,)] == 1
+    assert period.coefficients[(0,)][(1,)] == 8
+    assert period.coefficients[(0,)][(2,)] == 360
+
+    # L lives entirely within z_degree <= 2, theta_degree <= 4, its stated level.
+    recovered = period.find_annihilating_operators(z_degree=2, theta_degree=4)
+    assert len(recovered) == 1
+
+    # recovered[0] should be a scalar multiple of L - compare via the coefficient of the
+    # bare theta0^4 monomial, which is 1 in L.
+    ratio = recovered[0].operator.monomial_coefficient(L.theta_gens[0] ** 4) / L.operator.monomial_coefficient(
+        L.theta_gens[0] ** 4
+    )
+    assert ratio != 0
+    assert recovered[0].operator == ratio * L.operator