# RabbitFarm

### 2022-01-16

#### Primes and Pentagonals

The examples used here are from The Weekly Challenge problem statement and demonstrate the working solution.

## Part 1

Write a script to generate first 20 left-truncatable prime numbers in base 10.

### Solution

``````
use strict;
use warnings;

use boolean;
use constant N => 10_000;

sub sieve_atkin{
my(\$n) = @_;
my @primes = (2, 3, 5);
my \$upper_bound = int(\$n * log(\$n) + \$n * log(log(\$n)));
my @atkin = (false) x \$upper_bound;
my @sieve = (1, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 49, 53, 59);
for my \$x (1 .. sqrt(\$upper_bound)){
for(my \$y = 1; \$y <= sqrt(\$upper_bound); \$y+=2){
my \$m = (4 * \$x ** 2) + (\$y ** 2);
my @remainders;
@remainders = grep {\$m % 60 == \$_} (1, 13, 17, 29, 37, 41, 49, 53) if \$m <= \$upper_bound;
\$atkin[\$m] = !\$atkin[\$m] if @remainders;
}
}
for(my \$x = 1; \$x <= sqrt(\$upper_bound); \$x += 2){
for(my \$y = 2; \$y <= sqrt(\$upper_bound); \$y += 2){
my \$m = (3 * \$x ** 2) + (\$y ** 2);
my @remainders;
@remainders = grep {\$m % 60 == \$_} (7, 19, 31, 43) if \$m <= \$upper_bound;
\$atkin[\$m] = !\$atkin[\$m] if @remainders;
}
}
for(my \$x = 2; \$x <= sqrt(\$upper_bound); \$x++){
for(my \$y = \$x - 1; \$y >= 1; \$y -= 2){
my \$m = (3 * \$x ** 2) - (\$y ** 2);
my @remainders;
@remainders = grep {\$m % 60 == \$_} (11, 23, 47, 59) if \$m <= \$upper_bound;
\$atkin[\$m] = !\$atkin[\$m] if @remainders;
}
}
my @m;
for my \$w (0 .. (\$upper_bound / 60)){
for my \$s (@sieve){
push @m, 60 * \$w + \$s;
}
}
for my \$m (@m){
last if \$upper_bound < (\$m ** 2);
my \$mm = \$m ** 2;
if(\$atkin[\$m]){
for my \$m2 (@m){
my \$c = \$mm * \$m2;
last if \$c > \$upper_bound;
\$atkin[\$c] = false;
}
}
}
map{ push @primes, \$_ if \$atkin[\$_] } 0 .. @atkin - 1;
return @primes;
}

sub truncatable{
my(\$prime, \$primes) = @_;
return false if \$prime =~ m/0/;
my @truncatable = map { my \$p = substr(\$prime, -1 * \$_, \$_); grep {\$p == \$_} @{\$primes}} 1 .. length(\$prime);
return @truncatable == length(\$prime);
}

sub first_n_truncatable_primes{
my(\$n) = @_;
my @primes = sieve_atkin(N);
my @truncatable;
for my \$prime (@primes){
push @truncatable, \$prime if truncatable(\$prime, \@primes);
last if @truncatable == \$n;
}
return @truncatable;
}

MAIN:{
print join(", ", first_n_truncatable_primes(20)) . "\n";
}
``````

### Sample Run

``````
\$ perl perl/ch-1.pl
2, 3, 5, 7, 13, 17, 23, 37, 43, 47, 53, 67, 73, 83, 97, 113, 137, 167, 173, 197
``````

### Notes

First off, I am re-using the Sieve of Atkin code I wrote for a previous challenge. These challenges somewhat frequently have a prime number component so, if I get a chance, I'll compose that code into it's own module. If it weren't for the copy/paste of the Sieve of Atkin code then this solution would be very short! This sort of string manipulation is where Perl excels and the determination of whether a number is left truncatable takes only a few lines.

## Part 2

Write a script to find the first pair of Pentagon Numbers whose sum and difference are also a Pentagon Number.

### Solution

``````
use strict;
use warnings;

use constant N => 10_000;

sub n_pentagon_numbers{
my(\$n) = @_;
my @pentagon_numbers;
my \$x = 1;
my %h;
do{
my \$pentagon = \$x * (3 * \$x - 1) / 2;
push @pentagon_numbers, \$pentagon;
\$h{"\$pentagon"} = \$x;
\$x++;
}while(@pentagon_numbers < \$n);
return (\@pentagon_numbers, \%h);
}

sub pairs_pentagon{
my(\$n) = @_;
my(\$pentagons, \$lookup) = n_pentagon_numbers(N);
my @pairs;
for my \$x (0 .. @{\$pentagons} - 1){
for my \$y (0 .. @{\$pentagons} - 1){
unless(\$x == \$y){
my(\$sum, \$difference) = (\$pentagons->[\$x] + \$pentagons->[\$y], abs(\$pentagons->[\$x] - \$pentagons->[\$y]));
if(\$lookup->{\$sum} && \$lookup->{\$difference}){
my(\$s, \$t) = (\$x + 1, \$y + 1);
push @pairs, ["P(\$s)", "P(\$t)"]
}
}
last if @pairs == \$n;
}
last if @pairs == \$n;
}
return @pairs;
}

sub first_pair_pentagon{
return [pairs_pentagon(1)];
}

MAIN:{
print join(", ", @{first_pair_pentagon()->}) . "\n";
}
``````

### Sample Run

``````
\$ perl perl/ch-2.pl
P(1020), P(2167)
``````

### Notes

This second part of the challenge proceeds in mostly the same way as the first. We generate a large list of candidates and then search for those exhibiting the property in question. It is somewhat unexpected that the first pair of Pentagonal Numbers that have this property are so deeply located. Many times in these challenges the solution is emitted without quite as much searching!

## References

Challenge 147

Left Truncatable Primes

Pentagonal Numbers

posted at: 13:29 by: Adam Russell | path: /perl | permanent link to this entry