both_truncatable_primes_in_base.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 13 January 2018
# https://github.com/trizen

# Both-trucatable primes in a given base.

# Optimization:
#   there are fewer right-truncatable primes than are left-truncatable primes, therefore we generate
#   only the right-truncatable primes and check which ones of those are also left-truncatable primes.

# Maximum value for each base is given in the following OEIS sequence:
#   https://oeis.org/A323137

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

# Related sequences:
#  https://oeis.org/A076586 - Total number of right truncatable primes in base n.
#  https://oeis.org/A076623 - Total number of left truncatable primes (without zeros) in base n.
#  https://oeis.org/A323390 - Total number of primes that are both left-truncatable and right-truncatable in base n.
#  https://oeis.org/A323396 - Irregular array read by rows, where T(n, k) is the k-th prime that is both left-truncatable and right-truncatable in base n.

func is_left_truncatable_prime(n, base) {

    for (var r = base; r < n; r *= base) {
        n - r*idiv(n, r) -> is_prime || return false
    }

    return true
}

func generate_from_prefix(p, base, digits) {

    var seq = [p]

    digits.each {|d|
        var n = (p*base + d)
        if (n.is_prime) {
            seq << __FUNC__(n, base, digits).grep { is_left_truncatable_prime(_, base) }...
        }
    }

    return seq
}

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

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

    prime_digits.map  {|p| generate_from_prefix(p, base, digits)... }\
                .sort
}

for base in (3..15) {
    var a = both_truncatable_primes(base)
    say "There are #{'%3d' % a.len} both-truncatable primes in base #{'%2d' % base} where largest is #{a[-1]}"
}

__END__
There are   2 both-truncatable primes in base  3 where largest is 23
There are   3 both-truncatable primes in base  4 where largest is 11
There are   5 both-truncatable primes in base  5 where largest is 67
There are   9 both-truncatable primes in base  6 where largest is 839
There are   7 both-truncatable primes in base  7 where largest is 37
There are  22 both-truncatable primes in base  8 where largest is 1867
There are   8 both-truncatable primes in base  9 where largest is 173
There are  15 both-truncatable primes in base 10 where largest is 739397
There are   6 both-truncatable primes in base 11 where largest is 79
There are  35 both-truncatable primes in base 12 where largest is 105691
There are  11 both-truncatable primes in base 13 where largest is 379
There are  37 both-truncatable primes in base 14 where largest is 37573
There are  17 both-truncatable primes in base 15 where largest is 647
There are  22 both-truncatable primes in base 16 where largest is 3389
There are  12 both-truncatable primes in base 17 where largest is 631
There are  69 both-truncatable primes in base 18 where largest is 202715129
There are  12 both-truncatable primes in base 19 where largest is 211
There are  68 both-truncatable primes in base 20 where largest is 155863
There are  18 both-truncatable primes in base 21 where largest is 1283
There are  44 both-truncatable primes in base 22 where largest is 787817
There are  13 both-truncatable primes in base 23 where largest is 439
There are 145 both-truncatable primes in base 24 where largest is 109893629
There are  16 both-truncatable primes in base 25 where largest is 577
There are  47 both-truncatable primes in base 26 where largest is 4195880189
There are  20 both-truncatable primes in base 27 where largest is 1811
There are  77 both-truncatable primes in base 28 where largest is 14474071
There are  13 both-truncatable primes in base 29 where largest is 379