37282: Fix RegularSequence.subsequence for zero sequence

Author: cheuberg

src/sage/combinat/regular_sequence.py

@@ -762,6 +762,12 @@ def subsequence(self, a, b):
 
             sage: S.regenerated().subsequence(1, -4)
             2-regular sequence 0, 0, 0, 0, 1, 3, 6, 9, 12, 18, ...
+
+        Check that the zero sequence is handeled correctly (issue:`37282`)
+        ::
+
+            sage: Seq2.zero().subsequence(1, 1)
+            2-regular sequence 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
         """
         from itertools import chain
         from sage.rings.integer_ring import ZZ
@@ -835,14 +841,16 @@ def matrix_row(r, c):
             d, f = rule[r, c]
             return [self.mu[f] if d == j else zero_M for j in kernel]
 
+        # We explicitly set the ring when creating vectors in order to avoid
+        # problems with the zero sequence, see issue:`37282`.
         result = P.element_class(
             P,
             {r: Matrix.block([matrix_row(r, c) for c in kernel])
              for r in A},
-            vector(chain.from_iterable(
+            vector(P.coefficient_ring(), chain.from_iterable(
                 b.get(c, 0) * self.left
                 for c in kernel)),
-            vector(chain.from_iterable(
+            vector(P.coefficient_ring(), chain.from_iterable(
                 (self.coefficient_of_n(c, multiply_left=False) if c >= 0 else zero_R)
                 for c in kernel)))