Primes are positive integers that can only be divided by 1 and themselves. 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."
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[Category Concept] | [Arts and crafts of Tcl-Tk programming]