partial_sums_of_gpf.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 20 July 2020
# https://github.com/trizen

# Algorithm with sublinear time for computing:
#
#   Sum_{k=2..n} gpf(k)
#
# where:
#   gpf(k) = the greatest prime factor of k

# See also:
#   https://projecteuler.net/problem=642

func partial_sums_of_gpf(n) {

    var t = 0
    var s = n.isqrt

    s.each_prime {|p|
        t += p*p.smooth_count(idiv(n,p))
    }

    for (var p = s.next_prime; p <= n; p.next_prime!) {

        var u = idiv(n,p)
        var r = idiv(n,u)

        t += u*sum_primes(p,r)
        p = r
    }

    return t
}

for k in (1..7) {
    say "S(10^#{k}) = #{partial_sums_of_gpf(10**k)}"
}

__END__
S(10^1)  = 32
S(10^2)  = 1915
S(10^3)  = 135946
S(10^4)  = 10118280
S(10^5)  = 793111753
S(10^6)  = 64937323262
S(10^7)  = 5494366736156
S(10^8)  = 476001412898167
S(10^9)  = 41985754895017934
S(10^10) = 3755757137823525252
S(10^11) = 339760245382396733607
S(10^12) = 31019315736720796982142