_igamc_helper_series Class — pytorch Architecture

Source Code

aten/src/ATen/native/Math.h lines 685–710

template <typename scalar_t>
static scalar_t _igamc_helper_series(scalar_t a, scalar_t x) {
  // Compute igamc using DLMF 8.7.3 [igam1]. This is related to the series in
  // _igam_helper_series but extra care is taken to avoid cancellation.

  int n;
  scalar_t fac = 1;
  scalar_t sum = 0;
  scalar_t term, logx;
  static scalar_t MAXITER = 2000;
  static scalar_t MACHEP = std::is_same_v<scalar_t, double> ?
    1.11022302462515654042E-16 : 5.9604644775390625E-8;

  for (n = 1; n < MAXITER; n++) {
    fac *= -x / n;
    term = fac / (a + n);
    sum += term;
    if (std::fabs(term) <= MACHEP * std::fabs(sum)) {
        break;
    }
  }

  logx = std::log(x);
  term = -std::expm1(a * logx - std::lgamma(1+a));
  return term - std::exp(a * logx - std::lgamma(a)) * sum;
}