Returns the Ulam spiral.
m = number_ulamspiral ( n ) m = number_ulamspiral ( n , keepall )
a 1-by-1 matrix of floating point integers, must be odd and greater than 1
a 1-by-1 matrix of booleans, set to %t to keep all integers, set to %f to keep only primes (default keep=%f)
a n-by-n matrix of floating point integers
Returns the Ulam spiral.
http://en.wikipedia.org/wiki/Ulam_spiral