Thread Subject: calculate P value from r

Hi
I want to calculate P value from r without using function [r, P]=corrcoef() (because computation complexity issues):

for example:
v1 = [1 2 3 4];
v2 = [3 4 5 6];

r = v1 * ( v2' ) / sqrt( ( v1 * v1' ) * ( v2 * v2' ) );

now I want to get P from this r, how can I do it?

diego