logPmf
Returns the natural logarithm of the probability mass at k.
Computed directly in log-space as k * ln(p) - ln(k) + ln(a) where a = -1 / ln(1 - p).
Return
the natural log of the probability mass at k. Returns Double.NEGATIVE_INFINITY when k is outside the support.
Parameters
k
the positive integer at which to evaluate the log-mass.