HP 35s Scientific Calculator - Manual (Page 89)

Real–Number Functions 4-15
Probability Functions
Factorial
To calculate the factorial of a displayed non-negative integer x (0 ≤ x ≤ 253), press
 * (the right–shifted  key).
Gamma
To calculate the gamma function of a noninteger x, Γ(x), key in (x – 1) and press
 * . The x! function calculates Γ(x + 1). The value for x cannot be a negative
integer. 
Probability
Combinations
To calculate the number of possible sets of n items taken r at a time, enter n first,
 x , then r (nonnegative integers only). No item occurs more than once in a
set, and different orders of the same r items are not counted separately.
Permutations
To calculate the number of possible arrangements of n items taken r at a time, enter
n first,
{ , then r (nonnegative integers only). No item occurs more than
once in an arrangement, and different orders of the same r items are counted
separately.
Seed
To store the number in x as a new seed for the random number generator, press
  .
Random number generator
To generate a random number in the range 0 < x < 1, press   . (The
number is part of a uniformly–distributed pseudo–random number sequence. It
passes the spectral test of D. Knuth, The Art of Computer Programming, vol. 2,
Seminumerical Algorithms, London: Addison Wesley, 1981.)