> @raoulb , this is an unambiguous reference from a well reputable web
> site.

I know this site well. The problem is that one needs a login 
or pay for the pdf file. (For me, access is not a problem.)

> Anyone could find the article from the info there or search
> further for a free pdf online.

Right, this is true yes. First hit on google.

> Is this ok?

Well, I still prefer a real citation, that is a subset of the 
bibtex that is on the acm site. The subset should be at least
author, title, journal (& vol/issue) and year. BTW, I think
that we should cite any algorithm used in sympy this way.

There are very good examples in the doc of our poly module:

http://docs.sympy.org/0.7.2/modules/polys/literature.html

and also this one:

http://docs.sympy.org/0.7.2/modules/simplify/hyperexpand.html#references