197 Investigating the behaviour of a recursively defined sequence.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 03 March 2018
# License: GPLv3
# https://github.com/trizen

# https://projecteuler.net/problem=197

# Runtime: 0.183s

func a(n) is cached {
    return -1 if (n == 0)
    floor(2**(30.403243784 - a(n - 1)**2)) * 1e-9
}

for n in (1..Inf) {
    if ((a(n  ).round(-9) == a(n+2).round(-9)) &&
        (a(n+1).round(-9) == a(n+3).round(-9))) {
        say (a(n) + a(n+1))
        break
    }
}