Hello Again, Further to my earlier email, I've managed to narrow the problem down somewhat. It would seem that if the log() function is called before a call to gsl_eigen_symmmv with a matrix similar to one which has already been solved (this is a rough summary which may not be entirely accurate, but I think contains the relevant details) then that routine goes into an apparently infinite loop. I've attached a short piece of code which demonstrates this problem and I'd be interested to know whether it is a general one or is confined to systems like my own (or indeed, is due to a corrupt library and is only happening to me...). Many apologies if this is a known problem, or if there's some obvious reason why using the C libraries log function in conjunction with gsl isn't a reasonable thing to do.... Adam Johansen