sum_of_squarefree_k-almost_primes.sf

#!/usr/bin/ruby

# Author: Trizen
# Date: 16 August 2023
# https://github.com/trizen

# Sum of squarefree k-almost primes <= n.

# See also:
#   https://en.wikipedia.org/wiki/Almost_prime

func squarefree_almost_prime_sum(n, k) {

    if (k == 1) {
        return n.prime_sum
    }

    var total = 0

    func (m, lo, k, j = 0) {

        var hi = idiv(n,m).iroot(k)

        if (k == 2) {

            each_prime(lo, hi, {|p|
                j += p
                total += m*p*(prime_sum(idiv(n, m*p)) - j)
            })

            return nil
        }

        each_prime(lo, hi, {|p|
            j += p
            __FUNC__(m*p, p+1, k-1, j)
        })
    }(1, 2, k)

    return total
}

# Run some tests

for k in (1..10) {

    var upto = k.pn_primorial+1e5.irand

    var x = squarefree_almost_prime_sum(upto, k)
    var y = k.squarefree_almost_primes(upto).sum
    var z = k.squarefree_almost_prime_sum(upto)

    say "Testing: #{k} with n = #{upto} -> #{x}"

    assert_eq(x, y)
    assert_eq(x, z)
}

say ''

for k in (1..6) {
    say ("Sum of squarefree #{'%d' % k}-almost primes <= 10^n: ", 8.of {|n| squarefree_almost_prime_sum(10**n, k) })
}

__END__
Sum of squarefree 1-almost primes <= 10^n: [0, 17, 1060, 76127, 5736396, 454396537, 37550402023, 3203324994356]
Sum of squarefree 2-almost primes <= 10^n: [0, 16, 1620, 142800, 12671118, 1136592401, 102555164308, 9320987760054]
Sum of squarefree 3-almost primes <= 10^n: [0, 0, 286, 73624, 9415168, 1013522662, 104063487517, 10437330922560]
Sum of squarefree 4-almost primes <= 10^n: [0, 0, 0, 10524, 2434091, 379717493, 48777045480, 5695891373909]
Sum of squarefree 5-almost primes <= 10^n: [0, 0, 0, 0, 163260, 54077985, 10247415009, 1545644072692]
Sum of squarefree 6-almost primes <= 10^n: [0, 0, 0, 0, 0, 1404120, 761443872, 186672163072]