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walterreade/prob_unique.py

Created January 31, 2016 16:21
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prob_unique.py
from scipy import exp, log
from scipy.special import gammaln
def prob_unique(N, r):
""" If you have a set of N things to choose from, and take r samples,
the probability that all r samples are unique.
http://www.johndcook.com/blog/2016/01/30/general-birthday-problem
"""
return exp( gammaln(N+1) - gammaln(N-r+1) - r*log(N) )
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