130 Composites with prime repunit property -- v2.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 21 September 2019
# https://github.com/trizen

# https://projecteuler.net/problem=130

# Runtime: 0.389s

# Deceptive nonprimes are composite numbers n that are coprime to 30, satisfying 10^(n-1) == 1 (mod n).

# See also:
#   https://oeis.org/A000864

2..Inf -> lazy.grep {|n| n.is_composite }\
              .grep {|n| n.is_coprime(30) }\
              .grep {|n| powmod(10, n-1, n) == 1 }\
              .first(25).sum.say