Implement BigMath.gamma and BigMath.lgamma #451
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Implement gamma and lgamma with Spouge's approximation
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Implement gamma and lgamma with Spouge's approximation
https://en.wikipedia.org/wiki/Spouge%27s_approximation
It's relatively simple compared to other algorithm.
It's designed to converges in wide range of x without special care. This characteristic is important to calculate gamma for huge number:
BigMath.gamma(BigDecimal(10000000), 100)Other algorithms that I tried:
gamma(x)*f(x)with variousf(x)gamma(small_digits_x, prec)can be extremely fastgamma(huge_number, prec)is slow, so Lanczos or Spouge is necessary anyway