HP 42S - Manual (Page 16)

For a integer number we have Γ(n)=(n-1)! and Γ(n+1)=n!. The number in gamma function must be
real.
In this point HP-42S is different from 33S which has only one function for both things.
RAN: This is the random number generator which gives a pseudo-random number in 0≤x≤1.
SEED: A sequence of pseudo-random numbers always starts with a seed. If you repeat the seed the
sequence repeats. To enter a new seed just enter a number and press SEED.
If the seed is zero the calculator will generate another seed.