I have a piece of software that I have been working on for a while now. You can find it here. Since it is now at a stage where it could potentially be useful to other researchers, I decided to cover it by the GPL.
While most of it is code that I wrote myself, one set of functions, contained in butterworth.c and butterworth.h, are not - they are a library of functions I found on this website, and are covered by the GPLv2. The rest I have (tried to) cover with the GPLv3.
I suspect that having two versions of the GPL in the same project is not OK, so what is the proper way to cover the contents of butterworth.c in my project while still giving credit where it is due to the original programmers who wrote it?
I suppose, in addition, is there anything wrong/missing with the way I have set up the GPLv3 in the rest of the project? Currently it is just a comment block at the start of all .c files.