Fix conversion from SymbolicSeries to LaurentSeries

Author: user202729

src/sage/rings/laurent_series_ring.py

@@ -42,6 +42,7 @@
 from sage.rings.infinity import infinity
 from sage.rings.integer_ring import ZZ
 from sage.rings.laurent_series_ring_element import LaurentSeries
+from sage.structure.element import Expression
 from sage.structure.parent import Parent
 from sage.structure.unique_representation import UniqueRepresentation
 
@@ -542,6 +543,19 @@ def _element_constructor_(self, x, n=0, prec=infinity):
             sage: P.<x> = LaurentSeriesRing(QQ)
             sage: P({-3: 1})
             x^-3
+
+        Check that :issue:`39839` is fixed::
+
+            sage: var("x")
+            x
+            sage: f = (1/x+sqrt(x+1)).series(x, 5); f
+            1*x^(-1) + 1 + 1/2*x + (-1/8)*x^2 + 1/16*x^3 + (-5/128)*x^4 + Order(x^5)
+            sage: LaurentSeriesRing(QQ, "x")(f)
+            x^-1 + 1 + 1/2*x - 1/8*x^2 + 1/16*x^3 - 5/128*x^4 + O(x^5)
+            sage: PowerSeriesRing(QQ, "x")(f)
+            Traceback (most recent call last):
+            ...
+            ValueError: cannot return dense coefficient list with negative valuation
         """
         from sage.rings.fraction_field_element import FractionFieldElement
         from sage.rings.lazy_series import LazyPowerSeries, LazyLaurentSeries
@@ -589,6 +603,18 @@ def _element_constructor_(self, x, n=0, prec=infinity):
                     x = x.add_bigoh(self.default_prec())
             else:
                 x = x.add_bigoh(prec)
+        elif isinstance(x, Expression):
+            from sage.symbolic.expression import SymbolicSeries
+            if isinstance(x, SymbolicSeries):
+                v = x.default_variable()
+                if str(v) == self.variable_name():
+                    R = self.base_ring()
+                    g = self.gen()
+                    return sum(
+                            (R(a)*g**ZZ(e) for a, e in x.coefficients(v, sparse=True)), self.zero()
+                            ).add_bigoh(x.degree(x.default_variable()))
+                else:
+                    raise TypeError("can only convert series into ring with same variable name")
         return self.element_class(self, x, n).add_bigoh(prec)
 
     def random_element(self, algorithm='default'):

src/sage/symbolic/series_impl.pxi

@@ -199,57 +199,6 @@ cdef class SymbolicSeries(Expression):
         cdef Expression ex = new_Expression_from_GEx(self._parent, x)
         return ex
 
-    def coefficients(self, x=None, sparse=True):
-        r"""
-        Return the coefficients of this symbolic series as a list of pairs.
-
-        INPUT:
-
-        - ``x`` -- (optional) variable
-
-        - ``sparse`` -- boolean (default: ``True``); if ``False`` return a list
-          with as much entries as the order of the series
-
-        OUTPUT: depending on the value of ``sparse``,
-
-        - A list of pairs ``(expr, n)``, where ``expr`` is a symbolic
-          expression and ``n`` is a power (``sparse=True``, default)
-
-        - A list of expressions where the ``n``-th element is the coefficient of
-          ``x^n`` when ``self`` is seen as polynomial in ``x`` (``sparse=False``).
-
-        EXAMPLES::
-
-            sage: s = (1/(1-x)).series(x,6); s
-            1 + 1*x + 1*x^2 + 1*x^3 + 1*x^4 + 1*x^5 + Order(x^6)
-            sage: s.coefficients()
-            [[1, 0], [1, 1], [1, 2], [1, 3], [1, 4], [1, 5]]
-            sage: s.coefficients(x, sparse=False)
-            [1, 1, 1, 1, 1, 1]
-            sage: x,y = var("x,y")
-            sage: s = (1/(1-y*x-x)).series(x,3); s
-            1 + (y + 1)*x + ((y + 1)^2)*x^2 + Order(x^3)
-            sage: s.coefficients(x, sparse=False)
-            [1, y + 1, (y + 1)^2]
-        """
-        if x is None:
-            x = self.default_variable()
-        l = [[self.coefficient(x, d), d] for d in range(self.degree(x))]
-        if sparse:
-            return l
-
-        from sage.rings.integer_ring import ZZ
-        if any(not c[1] in ZZ for c in l):
-            raise ValueError("cannot return dense coefficient list with noninteger exponents")
-        val = l[0][1]
-        if val < 0:
-            raise ValueError("cannot return dense coefficient list with negative valuation")
-        deg = l[-1][1]
-        ret = [ZZ(0)] * int(deg+1)
-        for c in l:
-            ret[c[1]] = c[0]
-        return ret
-
     def power_series(self, base_ring):
         """
         Return the algebraic power series associated to this symbolic series.