Contents

goodfactors()

Usage:

goodfactors(n), n an integer scalar or vector

Keywords: general

goodfactors(n), where n is a vector of positive integers < 2^52 =
4503599627370496, returns an integer vector the same length as n whose
i-th element is the smallest integer >= n[i] having no prime factors
larger than 29.  It is designed to be used to choose legal lengths of
vectors to be Fourier transformed using rft(), hft() or cft().

goodfactors(n,m), where n is the same and m is a positive integer
scalar, does the same, except the i-th element of the result is the
smallest integer >= n[i] having no prime factors larger than m.

NOTE: Because the product of "unpaired" prime factors can't be too
large, it is not guaranteed that a value returned by goodfactors() is
actually a legal length for the Fourier transform functions.  For
example, goodfactors(255255) = 255255, but 255255 = 3*5*7*11*13*17 is
not a legal length.

Example:
  Cmd> goodfactors(vector(217,1409))
  (1)          220         1421

See also primefactors(), cft(), hft(), rft().