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#!/usr/bin/env perl
use strict;
use warnings;
use feature qw'say signatures';
no warnings qw'experimental::signatures';
use Data::Dumper;
use List::Util qw(all);
run() unless caller();
sub run() {
say join( ", ", left_truncatable_primes(20) );
}
sub left_truncatable_primes($n) {
my @primes = ();
for ( my $i = 2 ; @primes <= $n ; $i++ ) {
push @primes, $i if is_left_truncatable_prime($i);
}
return @primes;
}
sub is_left_truncatable_prime($x) {
return 0 if $x =~ /0/;
return 0 if !is_prime($x);
return 1 if length $x == 1;
return is_left_truncatable_prime( substr( $x, 1 ) );
}
sub is_prime($x) {
return 0 if $x < 2;
return 1 if $x < 4;
for ( my $i = 2 ; $i <= sqrt($x) ; $i++ ) {
return 0 if $x % $i == 0;
}
return 1;
}
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