dirichlet_hyperbola_method.sf

#!/usr/bin/ruby

# Simple implementation of Dirichlet's hyperbola method.

# This is one of the most beautiful formulas in mathematics!

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

func dirichlet_hyperbola_method(n,
    g = { _ },
    h = { _ },
    G = {|n| sum(1..n, g) },
    H = {|n| sum(1..n, h) }
) {

    var s = n.isqrt

    var A = sum(1..s, {|a|
        g(a) * H(floor(n/a))
    })

    var B = sum(1..s, {|b|
        h(b) * G(floor(n/b))
    })

    A + B - (G(s) * H(s))
}

func test_sum(n, g, h) {
    sum(1..n, {|k|
        k.divisors.sum {|d|
            g(d) * h(k/d)
        }
    })
}

func g(n) { n }
func h(n) { moebius(n) }

say 20.of { test_sum(_, g, h) }
say 20.of { dirichlet_hyperbola_method(_, g, h) }

__END__
[0, 1, 2, 4, 6, 10, 12, 18, 22, 28, 32, 42, 46, 58, 64, 72, 80, 96, 102, 120]
[0, 1, 2, 4, 6, 10, 12, 18, 22, 28, 32, 42, 46, 58, 64, 72, 80, 96, 102, 120]