[GiNaC-devel] Problem with power series

[Stefan Weinzierl]

Dear Chris,

attached is a patch (against GiNaC 1.3.7) which should fix the problem.

A few explanations: If one does a series expansion up to n of a product
A*B we have to know the ldegree of B in order to determine how far we have
to expand A (and vice versa).
Currently the implementation finds for each factor the ldegree first, by
expanding it to order 1,2,3,4,etc until a non-vanishing ldegree is found.

If a factor is a complicated zero, a non-vanishing ldegree is never found
and the algorithms loops for eternity.
In addition, this is quite inefficient, if a factor has a large ldegree:
Then this factor is expanded up to its ldegree, although we only asked for
an expansion up to n.

The patch splits the computation of the ldegrees into two stages:
In the first stage all negative ldegrees are computed.