right_truncatable_primes.sf

#!/usr/bin/ruby

# Generate right-truncatable primes that begin with a given prefix (without truncating the prefix).

# See also:
#   https://www.youtube.com/watch?v=azL5ehbw_24
#   https://en.wikipedia.org/wiki/Truncatable_prime

func right_truncatable_primes(p) {

    var seq = [p]

    for n in ([1, 3, 7, 9]) {
        var t = Number("#{p}#{n}")
        if (t.is_prime) {
            seq << right_truncatable_primes(t)...
        }
    }

    return seq
}

#
## Find the largest right-truncatable prime that begins with "n" and remains prime
## after each digit removed from the right, up to (but not including) the prefix "n".
#

for n in (1..15) {
    say ("a(#{n}) = ", right_truncatable_primes(n).max)
}

__END__
a(1) = 1979339339
a(2) = 29399999
a(3) = 37337999
a(4) = 4391339
a(5) = 59393339
a(6) = 6733997
a(7) = 73939133
a(8) = 839
a(9) = 9719
a(10) = 103997939939
a(11) = 113
a(12) = 12791333
a(13) = 13999133
a(14) = 149399
a(15) = 15797