Add a test

Author: kwankyu

src/sage/rings/function_field/khuri_makdisi.pyx

@@ -838,15 +838,35 @@ cdef class KhuriMakdisi_small(KhuriMakdisi_base):
             sage: q2 = G.point(pl2 - b)
             sage: G.point(af.divisor()) == q1.addflip(p2)
             True
+
+        Check that :issue:`40237` is fixed::
+
+            sage: K = GF(2)
+            sage: F.<x> = FunctionField(K)
+            sage: t = polygen(F)
+            sage: E.<y> = F.extension(t^3 + (x^2 + x + 1)*t^2 + (x^3 + x + 1)*t + x^5 + x^4)
+            sage: O = E.maximal_order()
+            sage: Oinf = E.maximal_order_infinite()
+            sage: D1 = -5 * O.ideal(x, y).divisor() + O.ideal(x + 1, y^2 + y + 1).divisor() \
+            ....:       + O.ideal(x^3 + x^2 + 1, y + x + 1).divisor()
+            sage: D2 = Oinf.ideal(1/x, y/x^2 + 1).divisor() - 5 * O.ideal(x, y).divisor() \
+            ....:       + O.ideal(x^4 + x^3 + 1, y + x).divisor()
+            sage: assert D1.degree() == 0
+            sage: assert D2.degree() == 0
+            sage: J = E.jacobian('km_small')
+            sage: JD1 = J(D1)
+            sage: JD2 = J(D2)
+            sage: JD1 + JD2 == JD2 + JD1
+            True
         """
         cdef int d0 = self.d0
         cdef int g = self.g
         cdef Matrix w1, w2, w3, w4, w5
 
         w1 = self.mu_image(wd1, wd2, self.mu_mat33, 4*d0 - g + 1)
-        # The row space of w2 represents H^0(O(3D_0 - D1 - D2)), whose dimension is
-        # at least d0 - g + 1, and hence the codimension is at most 2*d0. Thus,
-        # we cannot provide an expected_codim argument for mu_preimage.
+        # The row space of w2 represents H^0(O(3D_0 - D1 - D2)), whose
+        # dimension is at least d0 - g + 1. Hence the codimension is at most
+        # 2*d0, and we cannot provide an expected_codim argument for mu_preimage.
         w2 = self.mu_preimage(self.wV3, w1, self.mu_mat33)
         w3 = self.mu_preimage(self.wV2, w1, self.mu_mat42, 2*d0)
         # efficient than
@@ -896,8 +916,8 @@ cdef class KhuriMakdisi_small(KhuriMakdisi_base):
 
         w1 = self.mu_image(self.wV2, wd, self.mu_mat23, 4*d0 - g + 1)
         # The row space of w2 represents H^0(O(2D_0 - D)), whose dimension is
-        # at least d0 - g + 1, and hence the codimension is at most d0. Thus,
-        # we cannot provide an expected_codim argument for mu_preimage.
+        # at least d0 - g + 1. Hence the codimension is at most d0, and we
+        # cannot provide an expected_codim argument for mu_preimage.
         w2 = self.mu_preimage(self.wV3, w1, self.mu_mat23)
         # efficient than
         #   wf = matrix(w2[0])