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scipy.special.

# factorialk

scipy.special.factorialk(n, k, exact=False, extend='zero')

  Multifactorial of n of order k, n(!!…!).

  This is the multifactorial of n skipping k values. For example,

  > factorialk(17, 4) = 17!!!! = 17 * 13 * 9 * 5 * 1

  In particular, for any integer `n`, we have

  > factorialk(n, 1) = factorial(n)

  > factorialk(n, 2) = factorial2(n)

  Parameters:
    n int or float or complex (or array_like thereof)

      Input values for multifactorial. Non-integer values require `extend='complex'`. By default, the return value for `n < 0` is 0.

    n int or float or complex (or array_like thereof)

      Order of multifactorial. Non-integer values require `extend='complex'`.

    exact bool, optional

      If `exact` is set to True, calculate the answer exactly using integer arithmetic, otherwise use an approximation (faster, but yields floats instead of integers) Default is False.

    extend string, optional

      One of `'zero'` or `'complex'`; this determines how values `n<0` are handled - by default they are 0, but it is possible to opt into the complex extension of the multifactorial. This enables passing complex values, not only to `n` but also to `k`.

      Warning

      Using the `'complex'` extension also changes the values of the multifactorial at integers `n != 1 (mod k)` by a factor depending on both `k` and `n % k`, see below or [1].

  Returns:
    nf int or float or complex or ndarray

      Multifactorial (order `k`) of `n`, as integer, float or complex (depending on `exact` and `extend`). Array inputs are returned as arrays.

  Notes

  While less straight-forward than for the double-factorial, it’s possible to calculate a general approximation formula of n!(k) by studying `n` for a given remainder `r < k` (thus `n = m * k + r`, resp. `r = n % k`), which can be put together into something valid for all integer values `n >= 0` & `k > 0`:

      n!(k) = k ** ((n - r)/k) * gamma(n/k + 1) / gamma(r/k + 1) * max(r, 1)

  This is the basis of the approximation when `exact=False`.

  In principle, any fixed choice of `r` (ignoring its relation `r = n%k` to `n`) would provide a suitable analytic continuation from integer `n` to complex `z` (not only satisfying the functional equation but also being logarithmically convex, c.f. Bohr-Mollerup theorem) – in fact, the choice of `r` above only changes the function by a constant factor. The final constraint that determines the canonical continuation is `f(1) = 1`, which forces `r = 1` (see also [1]).:

      z!(k) = k ** ((z - 1)/k) * gamma(z/k + 1) / gamma(1/k + 1)

  References

  [1]

  Complex extension to multifactorial https://en.wikipedia.org/wiki/Double_factorial#Alternative_extension_of_the_multifactorial

  Examples

      >>> from scipy.special import factorialk
      >>> factorialk(5, k=1, exact=True)
      120
      >>> factorialk(5, k=3, exact=True)
      10
      >>> factorialk([5, 7, 9], k=3, exact=True)
      array([ 10,  28, 162])
      >>> factorialk([5, 7, 9], k=3, exact=False)
      array([ 10.,  28., 162.])