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 A357140 #5 Sep 15 2022 07:53:04 %S A357140 1,0,0,1,26,865,39268,2375965,185974145,18337523130,2227232055239, %T A357140 327003956573263,57121908284696448,11712143532463633752, %U A357140 2786114854266411595229,761208643373263480081077,236761580851204534640426709,83183218467008383189955036015 %N A357140 Number of n X n triangular (0,1)-matrices with exactly 2n entries equal to 1 and no zero rows or columns. %F A357140 a(n) = A137252(2n,n). %Y A357140 Cf. A137252. %K A357140 nonn %O A357140 0,5 %A A357140 _Alois P. Heinz_, Sep 14 2022