I would like to add the ability to check for Merseene Prime numbers to PDP-11 systems. The Lucas?Lehmer primality test at: http://en.wikipedia.org/wiki/Lucas%E2%80%93Lehmer_primality_test provides an almost trivial algorithm to use. When the values are small (less than 65K or when the original prime is less than 15), the calculation is able to use integers of just 4 bytes. I have managed to produce multi-precision subroutines for the PDP-11 which handle unsigned integers up to 10^160 which supports the calculation for integers up to 10^80. Unfortunately, the largest original prime which qualifies is 127, or not very large at all. What I really need is to figure out the calculations needed to perform: the Sch?nhage?Strassen algorithm: https://en.wikipedia.org/wiki/Sch%C3%B6nhage%E2%80%93Strassen_algorithm Does anyone have any hints how to actually turn the algorithm into code? When the values that must be squared are millions of 32-bit values long (really not too big, just 4 MB after all), the classical method to multiply 2 values which are each one million elements long takes one trillion multiplications plus many additions. The Sch?nhage?Strassen algorithm should reduce that to about one billion multiplications. Can anyone suggest where I can find something that explains the Sch?nhage?Strassen algorithm in a manner without all the FFT jargon getting in the way? Jerome Fine