The Geometric Distribution

Random: unsigned int gsl_ran_geometric (const gsl_rng * r, double p)
This function returns a random integer from the geometric distribution, the number of independent trials with probability p until the first success. The probability distribution for geometric variates is,

p(k) =  p (1-p)^(k-1)

for k >= 1.

Function: double gsl_ran_geometric_pdf (unsigned int k, double p)
This function computes the probability p(k) of obtaining k from a geometric distribution with probability parameter p, using the formula given above.