Perfect Number Generator

It calculates perfect even #'s with 2^(p-1)*(2^p - 1) where p and (2^p - 1)
%are prime. A subsequent run yields a faster run-time. The code is fully
%vectorized and hence cleaner than previous versions of the function.
%NB: if (2^p - 1) is a Mersenne prime then a perfect number has been found.
%
%SYNTAX: >> perf_finalv4(P) % where P is an integer scalar in
%useful inclusive range [2..31].
%i.e. run showing first 8 perfect numbers...
%>> p = perf_finalv4(31);
%>> [p(:,3)]
%
% ans =
%
% 6
% 28
% 496
% 8128
% 33550336
% 8589869056
% 137438691328
% 2.30584300813995e+018