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 A050204 #33 Feb 03 2021 21:49:27 %S A050204 1,2,21,1344,628080,2030271480,49846153939785,9350902053107858880, %T A050204 13807842729396124460629536,162526525004284727135887319788800, %U A050204 15464071313054035722739424623204673044920,12011244510939290610945342901003078567310643860160 %N A050204 a(n) is the number of n X n matrices of 0's, 1's and -1's in which the entries in each row or column sum to 1. %Y A050204 Cf. A005130 (Robbins numbers). %Y A050204 Cf. A229165 (entries in each row or column sum to 1 and there are no adjacent -1's or 1's in any row or column). %Y A050204 Cf. A172645 (case row and column sums are zero). %K A050204 nonn %O A050204 1,2 %A A050204 _Eric W. Weisstein_ %E A050204 Definition corrected by Murphy Waggoner and _R. H. Hardin_, Sep 24 2013 %E A050204 a(6)-a(8) from _R. H. Hardin_, Sep 25 2013 %E A050204 a(9)-a(12) from _Andrew Howroyd_, Feb 03 2021