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- #!/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
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