sidef-scripts/Math/infinitary_divisors.sf

#!/usr/bin/ruby

# Generate the infinitary divisors (or i-divisors) of n.

# See also:
#   https://oeis.org/A049417
#   https://oeis.org/A077609

func infinitary_divisors(n) {

    var d = [1]

    for p,e in (n.factor_exp) {

        var r = 1

        d << gather {
            for j in (1..e) {
                r *= p
                take(d.map {|u| u*r }...) if (e&j == j)
            }
        }...
    }

    return d.sort
}

for n in (1..20) {
    say "i-divisors of #{n}: #{infinitary_divisors(n)}"
    assert_eq(infinitary_divisors(n).len, n.isigma0)
    assert_eq(infinitary_divisors(n).sum, n.isigma)
    assert_eq(infinitary_divisors(n),     n.idivisors)
}

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