From b773294dd380ae81056c3d37b647500aae738ea1 Mon Sep 17 00:00:00 2001
From: "Vladimir V. Kisil" <V.Kisilv@leeds.ac.uk>
Date: Mon, 27 Jul 2020 21:03:42 +0200
Subject: [PATCH] Fix bug in power::expand() with the overall coefficient.

The negative sign of overall coefficient had been lost if a
power base contained some sign-definite factors.
---
 check/exam_paranoia.cpp | 23 +++++++++++++++++++++++
 ginac/power.cpp         |  5 +++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/check/exam_paranoia.cpp b/check/exam_paranoia.cpp
index 52466b05..7a64e52c 100644
--- a/check/exam_paranoia.cpp
+++ b/check/exam_paranoia.cpp
@@ -651,6 +651,28 @@ unsigned exam_paranoia26()
 	return 0;
 }
 
+// Bug in power expansion
+unsigned exam_paranoia27()
+{
+	unsigned result = 0;
+	symbol x("x"), y("y"), a("a");
+	possymbol s("s"), t("t");
+	exmap pwrs =
+	  { {pow((x+1)*(y-2)*(s-3)*(t+4), a), pow((x+1)*(y-2)*(s-3), a)*pow(t+4, a)},
+	    {pow(2*(x+1)*(y-2)*(s-3)*(t+4), a), pow(2,a)*pow((x+1)*(y-2)*(s-3), a)*pow(t+4, a)},
+	    {pow(-(x+1)*(y-2)*(s-3)*(t+4), a), pow(-(x+1)*(y-2)*(s-3), a)*pow(t+4, a)},
+	    {pow(-2*(x+1)*(y-2)*(s-3)*(t+4), a), pow(2,a)*pow(-(x+1)*(y-2)*(s-3), a)*pow(t+4, a)} };
+
+	for (auto e : pwrs) {
+		if (! (e.first.expand()).is_equal(e.second) ) {
+			clog << "power expansion of " << e.first << " produces error.\n";
+			++result;
+		}
+	}
+
+	return result;
+}
+
 unsigned exam_paranoia()
 {
 	unsigned result = 0;
@@ -684,6 +706,7 @@ unsigned exam_paranoia()
 	result += exam_paranoia24();  cout << '.' << flush;
 	result += exam_paranoia25();  cout << '.' << flush;
 	result += exam_paranoia26();  cout << '.' << flush;
+	result += exam_paranoia27();  cout << '.' << flush;
 	
 	return result;
 }
diff --git a/ginac/power.cpp b/ginac/power.cpp
index 19b31034..ba7a66f4 100644
--- a/ginac/power.cpp
+++ b/ginac/power.cpp
@@ -809,9 +809,10 @@ ex power::expand(unsigned options) const
 		ex coeff=(possign? _ex1 : _ex_1);
 		if (m.overall_coeff.info(info_flags::positive) && m.overall_coeff != _ex1)
 			prodseq.push_back(pow(m.overall_coeff, exponent));
-		else if (m.overall_coeff.info(info_flags::negative) && m.overall_coeff != _ex_1)
+		else if (m.overall_coeff.info(info_flags::negative) && m.overall_coeff != _ex_1) {
 			prodseq.push_back(pow(-m.overall_coeff, exponent));
-		else
+			coeff = -coeff;
+		} else
 			coeff *= m.overall_coeff;
 
 		// If positive/negative factors are found, then extract them.
-- 
2.44.0