conditional_euler_totient_function.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 04 July 2018
# https://github.com/trizen

# Find the number of k = 1..n for which GCD(n,k) satisfies a certain condition (e.g.:
# GCD(n,k) is a prime number), using the divisors of `n` and the Euler totient function.

# See also:
#   https://oeis.org/A117494 -- Number of k = 1..n for which GCD(n, k) is a prime
#   https://oeis.org/A116512 -- Number of k = 1..n for which GCD(n, k) is a power of a prime
#   https://oeis.org/A206369 -- Number of k = 1..n for which GCD(n, k) is a square
#   https://oeis.org/A078429 -- Number of k = 1..n for which GCD(n, k) is a cube
#   https://oeis.org/A063658 -- Number of k = 1..n for which GCD(n, k) is divisible by a square greater than 1

func foo(n, condition) {        # slow
    1..n -> count {|k|
        condition(gcd(n,k))
    }
}

func bar(n, condition) {        # fast
    n.divisors.sum {|d|
        condition(d) ? euler_phi(n/d) : 0
    }
}

func condition(d) {
    d.is_prime
}

say 25.of {|n| foo(n, condition) }
say 25.of {|n| bar(n, condition) }

assert_eq(
    100.of {|n| foo(n, condition) },
    100.of {|n| bar(n, condition) }
)