Primes are positive integers that can only be divided (without remainder) by 1 and themselves. Examples from RS's collection:
1234567891 987654321987654329
Here's a very simple prime checker, which returns 1 for primes, else the first found divisor (and the quotient):
proc primetest n {
set max [expr wide(sqrt($n))]
if {$n%2==0} {return [list 2 [expr $n/2]]}
for {set i 3} {$i<=$max} {incr i 2} {
if {$n%$i==0} {return [list $i [expr $n/$i]]}
}
return 1
}AM commented in the Tcl chatroom: "The trick: divide by the first ten or twenty primes, after that: numbers of the form 6n+1 and 6n+5 may act as divisors. If you use only numbers of the form I mentioned, you gain about 50% with no loss of information."
The first 6 million primes: http://www.geocities.com/ResearchTriangle/Thinktank/2434/prime/primenumbers.html
Mathworld: http://mathworld.wolfram.com/PrimeNumber.html