sum_of_cubes_function_recursive.sf

#!/usr/bin/ruby

# Author: Daniel "Trizen" Șuteu
# Date: 15 August 2021
# https://github.com/trizen

# Count the number of ways or representing n as a sum of k absolute cubes.

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

# OEIS sequences:
#   https://oeis.org/A175362 -- Number of integer pairs (x,y) satisfying |x|^3 + |y|^3 = n, -n <= x,y <= n.
#   https://oeis.org/A175365 -- Number of integer triples (x,y,z) satisfying |x|^3+|y|^3+|z|^3 = n, -n <= x,y,z <= n.
#   https://oeis.org/A175368 -- Number of integer 4-tuples (x,y,z,u) satisfying |x|^3+|y|^3+|z|^3+|u|^3 = n, -n <= x,y,z,u <= n.

func is_sum_of_two_cubes(n) {   # OEIS: A004999

    var L = iroot(n-1, 3)+1
    var U = iroot(4*n, 3)

    n.divisors.each {|m|
        if ((L <= m) && (m <= U)) {
            var l = (m*m - n/m)
            l.is_div(3) || next
            l = idiv(l, 3)
            is_square(m*m - 4*l) && return true
        }
    }

    return false
}

func cubes_r(n, k=2) is cached {

    return 1 if (n == 0)
    return 0 if (k <= 0)

    return (n.is_cube ? 1 : 0) if (k == 1)

    if (k == 2) {
        is_sum_of_two_cubes(n) || return 0
    }

    var count = 0

    for a in (0 ..  n.iroot(3)) {
        if (k > 2) {
            count += (a.is_zero ? 1 : 2)*__FUNC__(n - a**3, k-1)
        }
        elsif (n - a**3 -> is_cube) {
            count += (a.is_zero ? 1 : 2)*(n - a**3 -> is_zero ? 1 : 2)
        }
    }

    return count
}

for k in (0..9) {
    say ("k = #{k}: ", 20.of { cubes_r(_, k) })
}

__END__
k = 0: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
k = 1: [1, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
k = 2: [1, 4, 4, 0, 0, 0, 0, 0, 4, 8, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0]
k = 3: [1, 6, 12, 8, 0, 0, 0, 0, 6, 24, 24, 0, 0, 0, 0, 0, 12, 24, 0, 0]
k = 4: [1, 8, 24, 32, 16, 0, 0, 0, 8, 48, 96, 64, 0, 0, 0, 0, 24, 96, 96, 0]
k = 5: [1, 10, 40, 80, 80, 32, 0, 0, 10, 80, 240, 320, 160, 0, 0, 0, 40, 240, 480, 320]
k = 6: [1, 12, 60, 160, 240, 192, 64, 0, 12, 120, 480, 960, 960, 384, 0, 0, 60, 480, 1440, 1920]
k = 7: [1, 14, 84, 280, 560, 672, 448, 128, 14, 168, 840, 2240, 3360, 2688, 896, 0, 84, 840, 3360, 6720]
k = 8: [1, 16, 112, 448, 1120, 1792, 1792, 1024, 272, 224, 1344, 4480, 8960, 10752, 7168, 2048, 112, 1344, 6720, 17920]
k = 9: [1, 18, 144, 672, 2016, 4032, 5376, 4608, 2322, 800, 2016, 8064, 20160, 32256, 32256, 18432, 4752, 2016, 12096, 40320]