unitary_powerfree_divisors.sf

#!/usr/bin/ruby

# A simple algorithm for generating the unitary k-powerfree divisors of a given number n.

# See also:
#   https://oeis.org/A092261
#   https://mathworld.wolfram.com/Powerfree.html
#   https://en.wikipedia.org/wiki/Unitary_divisor

func unitary_powerfree_divisors(n, k=2) {

    var d  = [1]

    for p,e in (n.factor_exp) {
        e < k || next
        var r = p**e
        d << d.map {|t| t*r }...
    }

    return d.sort
}

for n in (1..20) {
    say "unitary squarefree divisors of #{n} = #{unitary_powerfree_divisors(n, 2)}"

    assert_eq(unitary_powerfree_divisors(n, 2), n.udivisors.grep { .is_powerfree(2) })
    assert_eq(unitary_powerfree_divisors(n, 3), n.udivisors.grep { .is_powerfree(3) })
    assert_eq(unitary_powerfree_divisors(n, 4), n.udivisors.grep { .is_powerfree(4) })
}

__END__
unitary squarefree divisors of 1 = [1]
unitary squarefree divisors of 2 = [1, 2]
unitary squarefree divisors of 3 = [1, 3]
unitary squarefree divisors of 4 = [1]
unitary squarefree divisors of 5 = [1, 5]
unitary squarefree divisors of 6 = [1, 2, 3, 6]
unitary squarefree divisors of 7 = [1, 7]
unitary squarefree divisors of 8 = [1]
unitary squarefree divisors of 9 = [1]
unitary squarefree divisors of 10 = [1, 2, 5, 10]
unitary squarefree divisors of 11 = [1, 11]
unitary squarefree divisors of 12 = [1, 3]
unitary squarefree divisors of 13 = [1, 13]
unitary squarefree divisors of 14 = [1, 2, 7, 14]
unitary squarefree divisors of 15 = [1, 3, 5, 15]
unitary squarefree divisors of 16 = [1]
unitary squarefree divisors of 17 = [1, 17]
unitary squarefree divisors of 18 = [1, 2]
unitary squarefree divisors of 19 = [1, 19]
unitary squarefree divisors of 20 = [1, 5]