Berhard Strohmeier answered the following question:

BS> >Does anyone know the algorithm, or the characteristics, of

BS> >the generator used for the ? operator in J?

BS> It's the Lewis, Miller, Goodman generator, the same as in APL. It is

BS> the multiplicative congruential generator x_{n+1)=7^5 x_n mod 2^31 -

BS> 1

BS> For characteristics look up the following article:

BS> Park, Miller: Random Number Generators: Good ones are hard to find

BS> Communications of the ACM, Vol 31, No 10, October 1988

Even though it does not refer to J, John Maddox's editorial in Nature

may be of interest to those concerned about Random Number Generators:

The poor quality of random numbers

Nature vol.372 (1 December 1994) p.403