left_truncatable_primes_in_base.sf

#!/usr/bin/ruby

# Generate left-truncatable primes in a given base.

# See also:
#   https://oeis.org/A103443 -- Largest left-truncatable prime in base n (given in decimal).
#   https://oeis.org/A076623 -- Total number of left truncatable primes (without zeros) in base n.
#   https://www.youtube.com/watch?v=azL5ehbw_24
#   https://en.wikipedia.org/wiki/Truncatable_prime
#   https://rosettacode.org/wiki/Find_largest_left_truncatable_prime_in_a_given_base

func generate_from_suffix(p, base) {

    var seq = [p]

    for n in (1 ..^ base) {
        var t = [p..., n]
        if (is_prime(digits2num(t, base))) {
            seq << __FUNC__(t, base)...
        }
    }

    return seq
}

func left_truncatable_primes(base) {     # finite sequence for each base

    var prime_digits = (base-1 -> primes)   # prime digits < base

    prime_digits.map  {|p| generate_from_suffix([p], base)... }\
                .map  {|t| digits2num(t, base) }\
                .sort
}

for n in (3..9) {
    var ltp = left_truncatable_primes(n)
    say ("There are #{'%4d' % ltp.len} left-truncatable primes in base #{'%2d' % n}, where largest is #{ltp.max}")
}

__END__
There are    3 left-truncatable primes in base  3, where largest is 23
There are   16 left-truncatable primes in base  4, where largest is 4091
There are   15 left-truncatable primes in base  5, where largest is 7817
There are  454 left-truncatable primes in base  6, where largest is 4836525320399
There are   22 left-truncatable primes in base  7, where largest is 817337
There are  446 left-truncatable primes in base  8, where largest is 14005650767869
There are  108 left-truncatable primes in base  9, where largest is 1676456897
There are 4260 left-truncatable primes in base 10, where largest is 357686312646216567629137
There are   75 left-truncatable primes in base 11, where largest is 2276005673