expr: fixed limit/series incorrect simplification, added regression test

.mailmap

@@ -975,6 +975,7 @@ Michael Mayorov <marchael@kb.csu.ru>
 Michael Mueller <michaeldmueller7@gmail.com>
 Michael S. Hansen <michael.hansen@nih.gov>
 Michael Sparapany <msparapa@purdue.edu>
+Michael Yang <yangmi@umich.edu>
 Michael Zingale <michael.zingale@stonybrook.edu>
 Michal Grňo <m93a.cz@gmail.com> Michal Grno <m93a.cz@gmail.com>
 Michał Radwański <enedil.isildur@gmail.com>

sympy/core/expr.py

@@ -3068,7 +3068,8 @@ def series(self, x=None, x0=0, n=6, dir="+", logx=None, cdir=0):
 
             try:
                 from sympy.simplify.radsimp import collect
-                return collect(s1, x) + o
+                from sympy.simplify.ratsimp import ratsimp
+                return collect(ratsimp(s1), x) + o
             except NotImplementedError:
                 return s1 + o
 

sympy/core/tests/test_expr.py

@@ -2286,3 +2286,10 @@ def test_format():
 
 def test_issue_24045():
     assert powsimp(exp(a)/((c*a - c*b)*(Float(1.0)*c*a - Float(1.0)*c*b)))  # doesn't raise
+
+def test_issue_9173():
+    p_0, p_1, p_2, p_3, b_0, b_1, b_2 = symbols('p_0:4, b_0:3')
+    Q = (p_0 + (p_1 + (p_2 + p_3/y)/y)/y)/(1 + ((p_3/(b_0*y) +
+        (b_0*p_2 - b_1*p_3)/b_0**2)/y + (b_0**2*p_1 - b_0*b_1*p_2 -
+            p_3*(b_0*b_2 - b_1**2))/b_0**3)/y)
+    assert Q.series(y, n=3) == b_2*y**2 + b_1*y + b_0 + O(y**3)