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I took your ideas for avoiding re-testing things and revamped my sieve. This generates all the answers that don't require factorial on anything higher than whatever you set (350 gets me all solutions for 1..30). It runs in about 5 seconds on my 1 GHz PC with that setting.
use strict; use warnings; use bigint; # How high a number are you willing to factorialize? my $factorial_limit = 999; # Pre-cache them to save on calls my @factorials = do { my $last_fact = 1; (0, 1, map { $last_fact *= $_ + } 2..$factorial_limit) }; # What's the biggest number you want to print? my $print_up_to = 200; # Each element of @formulas is an an op-string: # a string of Fs and Ss, indicating factorial # and square root, in order applied to get the # index my %formulas = (3 => '3'); my @unapplied = ([3, '3']); while (@unapplied) { my ($v, $ops) = @{shift(@unapplied)}; # Apply factorial if ($v <= $factorial_limit) { my $fv = $factorials[$v]; my $fops = $ops . 'F'; if (not exists $formulas{$fv}) { push @unapplied, [$fv, $fops]; $formulas{$fv} = $fops; } } # Apply sqrt $v = int(sqrt($v)); $ops .= 'S'; if (not exists $formulas{$v}) { push @unapplied, [$v, $ops]; $formulas{$v} = $ops; } } for (sort { $a <=> $b } keys %formulas) { last if $_ > $print_up_to; print "$_: $formulas{$_}\n"; }
Update: pregenerated factorials to save time; also put in a limit on how high to print, so the huge numbers don't show up. Even taking factorials as high as 999 (takes 30-40 seconds), there's no solution for 38.

Caution: Contents may have been coded under pressure.

In reply to Re^2: Challenge: Chasing Knuth's Conjecture by Roy Johnson
in thread Challenge: Chasing Knuth's Conjecture by kvale

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