Where do random numbers come from really? 
 
 
Uniformly distributed random variates Xi = remainder(aXi-1 / m)
For example,  a= 75           m= 231 -1
Given two Xj Xk  such  uniform random variates,
Normally distributed random variates can be made 
Xi =  sqrt(-2log(Xj)) cos(2pXk)     (NR, Press et al. p. 279-89)