This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A264040 #20 Feb 16 2025 08:33:27 %S A264040 1,12,181440,10461394944000,7755605021665492992000000, %T A264040 185996663394950608733999724075417600000000, %U A264040 304140932017133780436126081660647688443776415689605120000000000,63443466092942082051716694667580740401432758087272596099400947187607352115200000000000000 %N A264040 Number of possible permutations of the n X n generalization of the sliding block 15-puzzle. %C A264040 For n > 1, of the permutations that can be reached by disassembling the puzzle and replacing the tiles, exactly half of them can be reached by sliding the tiles. %H A264040 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/15Puzzle.html">15 Puzzle</a> %F A264040 a(1) = 1; a(n) = (n^2)!/2 for n > 1. %e A264040 a(4) = 10461394944000 because the standard 4 X 4 version of the 15-puzzle has exactly 10461394944000 permutations that can be reached by sliding the tiles. %t A264040 a[n_] := If[n == 1, 1, (n^2)!/2] %Y A264040 Cf. A087725, A090031, A088020. %K A264040 easy,nonn %O A264040 1,2 %A A264040 _Ben Whitmore_, Nov 01 2015 %E A264040 a(1) added by _Franklin T. Adams-Watters_, Nov 11 2015