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- #!/usr/bin/ruby
- # Let a(p) be the largest left-truncatable prime that ends in "p" (without truncating "p").
- # The largest value of a(p) for 2 < p < 10 is:
- # a(7) = 357686312646216567629137
- # The first three terms such that a(p) > a(7) are:
- # a(4111) = 689787684933263366794111
- # a(13367) = 833757579699383379513367
- # a(28517) = 5391333548376627357228517
- # See also:
- # https://www.youtube.com/watch?v=azL5ehbw_24
- # https://en.wikipedia.org/wiki/Truncatable_prime
- func left_truncatable_primes(p) {
- var seq = [p]
- for n in (1..9) {
- var t = Number("#{n}#{p}")
- if (t.is_prime) {
- seq << left_truncatable_primes(t)...
- }
- }
- return seq
- }
- say left_truncatable_primes(3).max #=> 36484957213536676883
- say left_truncatable_primes(7).max #=> 357686312646216567629137
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