recursive_gcd_prime_generator.sf

#!/usr/bin/ruby

# An interesting recursive function, involving the gcd() function for generating prime numbers.

# If R(n) = 2*n + 2, then n+1 is prime. (conjectured?)

func R((1)) { 7 }

func R(n) is cached {
    R(n-1) + gcd(R(n-1), n)
}

each(1..10000, { |n|
    if (R(n) == (2*n + 2)) {
        say n+1
    }
})

__END__
[5, 11, 23, 47, 101, 233, 467, 941, 1889, 3779, 7559]