prog.pl

#!/usr/bin/perl

# Efficient algorithm for generating sub-unit squares.

# A sub-unit square is a square number that remains a square after having a 1 subtracted from each digit in the square.

# See also:
#   https://oeis.org/A061844
#   https://rosettacode.org/wiki/Sub-unit_squares

# Checked up to (10^167 - 1)/9.
# Checked all n <= 352 with sigma0() <= 10^8.
# Checked all even n <= 662 (except 664) with sigma0() <= 10^8.

use 5.036;
use ntheory qw(:all);

use Math::GMPz;
use Math::Prime::Util::GMP qw();

use Math::Sidef  qw();
use Math::AnyNum qw();

$Sidef::Types::Number::Number::VERBOSE      = '1';
$Sidef::Types::Number::Number::USE_FACTORDB = '1';

sub difference_of_two_squares_solutions ($n, $callback) {    # solutions x to x^2 - y^2 = n

    state $limit = Math::GMPz::Rmpz_init();
    Math::GMPz::Rmpz_sqrt($limit, $n);

    #Math::Sidef::sigma0($n) <= 1e8 or return;

    my @factors = map { "$_" } grep { $$_ <= $limit } Math::Sidef::factor($n);

    state $p = Math::GMPz::Rmpz_init();
    state $q = Math::GMPz::Rmpz_init();
    state $t = Math::GMPz::Rmpz_init();

    foreach my $k (0 .. scalar(@factors)) {

        my $comb = binomial(scalar(@factors), $k);

        if ($comb > 1e5) {
            say "Combinations: (", scalar(@factors), ", $k) == $comb";
        }

        forcomb {
            Math::GMPz::Rmpz_set_str($p, Math::Prime::Util::GMP::vecprod(@factors[@_]), 10);

            if (Math::GMPz::Rmpz_cmp($p, $limit) <= 0) {

                Math::GMPz::Rmpz_div($q, $n, $p);
                Math::GMPz::Rmpz_add($t, $p, $q);

                if (Math::GMPz::Rmpz_even_p($t)) {
                    Math::GMPz::Rmpz_div_2exp($t, $t, 1);
                    $callback->($t);
                }
            }
        }
        scalar(@factors), $k;
    }
}

my $N    = 1000;       # how many terms to compute
my %seen = (1 => 1);

my $index = 1;
say($index, ': ', 1);

for (my $n = 1 ; ; ++$n) {

    my $r = (Math::GMPz->new(10)**$n - 1) / 9;

    my $t    = Math::GMPz::Rmpz_init();
    my $xsqr = Math::GMPz::Rmpz_init();

    difference_of_two_squares_solutions(
        $r,
        sub ($x) {

            Math::GMPz::Rmpz_mul($xsqr, $x, $x);

            my $str = Math::GMPz::Rmpz_get_str($xsqr, 10);

            return if (substr($str, 0, 1) eq '1');
            return if (index($str, '0') != -1);

            my @d = split(//, $str);

            Math::GMPz::Rmpz_set_str($t, join('', map { $_ - 1 } @d), 10);
            Math::GMPz::Rmpz_perfect_square_p($t) || return;

            if (!$seen{$xsqr}++) {
                say(++$index, ': ', $xsqr);
                exit if ($index >= $N);
            }
        }
    );
}

__END__
1: 1
2: 36
3: 3136
4: 24336
5: 5973136
6: 71526293136
7: 318723477136
8: 264779654424693136
9: 24987377153764853136
10: 31872399155963477136
11: 58396845218255516736
12: 517177921565478376336
13: 252815272791521979771662766736
14: 518364744896318875336864648336
15: 554692513628187865132829886736
16: 658424734191428581711475835136
17: 672475429414871757619952152336
18: 694688876763154697414122245136
19: 711197579293752874333735845136
20: 975321699545235187287523246336
21: 23871973274358556957126877486736
22: 25347159162241162461433882565136
23: 34589996454813135961785697637136
24: 2858541763747552538199941619545257144336
25: 214785886789716796533667464535274377236736
26: 233292528132679183463629157143235636286736
27: 244671849793441155421899813243325528686736
28: 271571567929448516411695557685613529966736
29: 322388381596588665613523969581347191316736
30: 385414415625146742626881165526237149942336
31: 494827714874767379344736911473964125592336
32: 729191918879671448289782722539515523333136
33: 739265858539339252384919139328667324488336
34: 451616391374794616993675837721511769881724292768597136