factor and large prime numbers

[Sami Kerola]

Hello,

I were curious how quickly factor will process prime numbers, and
found something rather strange. For example these five can be computed
quite quickly.

time factor 10333147966386144929666651337523199999999
time factor 371993326789901217467999448150835199999999
time factor 13763753091226345046315979581580902399999999
time factor 523022617466601111760007224100074291199999999
time factor 20397882081197443358640281739902897356799999999

But much smaller numbers will take ages to give results (to be honest
I gave up).

time factor 8683317618811886495518194401279999999
time factor 295232799039604140847618609643519999999

Any idea what is going on?

--
Sami Kerola
http://www.iki.fi/kerolasa/