Dear Feng, Thanks for reporting this (rather severe) issue. I am attaching a patch to fix it. It also includes additional GiNaC self-check. Best wishes, Vladimir PS. The previous not yet committed bugfix is attached as well. -- Vladimir V. Kisil http://www.maths.leeds.ac.uk/~kisilv/ Book: Geometry of Mobius Maps https://doi.org/10.1142/p835 Soft: Geometry of cycles http://moebinv.sourceforge.net/ Jupyter notebooks: https://github.com/vvkisil/MoebInv-notebooks ________________________________________ From: GiNaC-list <ginac-list-bounces@ginac.de> on behalf of Feng Feng <f.feng@outlook.com> Sent: 27 July 2020 02:23 To: ginac-list@ginac.de Subject: [GiNaC-list] 回复: issue about series involving positive symbol I got a more compact case for this issue, by changing the in1 and in2 to ex in1 = reader("t^2/(3*m^2-t*m^2)/2"); ex in2 = reader("-(m^(-2)*(-3+t)^(-1)*t^2/2)"); sometimes we get the output: in1-in2 = 0 diff = 0 while, for some other times, we get the output: in1-in2 = 0 diff = 96*log(1/48) it seems that the sign of the log term has been changed. Best regards! Feng