primeCheckFermat

rwm: I thought the primes page was getting too crowded so I added this as a new page. My intent on adding this was to help teach the math and provide a simple Tcl implementation of the algorithm. (It might belong in a cookbook of useful recipes??)

Warning - It only checks 'a'=2 which will include some 'liars'. It could be changed to check other a values as well.

References:

1. http://en.wikipedia.org/wiki/Fermat_primality_test

Dependencies:

1. binaryModExpo - a modular exponentiation function (I like the one at the bottom of RSA in Tcl)

proc primeCheckFermat {p} {
    ## based on fermats little theorem
    ## a**(p-1) == 1
    ## for a in the interval [1,p-1]

    if {[binaryModExpo 2 $p $p] == 2} {
        # possible prime (or fermat liar)
        return 1
    } else {
        #puts stderr "failed fermat check ..."
        # not possible
        return 0
    }
}