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 A005163 M1500 #31 Sep 22 2023 16:13:35 %S A005163 1,2,5,16,67,368,2630,24376,293770,4610624,94080653,2492747656, %T A005163 85827875506,3842929319936,223624506056156,16901839470598576, %U A005163 1659776507866213636,211853506422044996288,35137231473111223912310,7569998079873075147860464 %N A005163 Number of alternating sign n X n matrices that are symmetric about a diagonal. %C A005163 Robbins's paper does not give a formula for this sequence. On the contrary he states: "Apparently these numbers do not factor into small primes, so a simple product formula seems unlikely. Of course this does not rule out other very simple formulas, but these would be more difficult to discover (let alone prove)." As far as I know no formula is currently known. - Herman Jamke (hermanjamke(AT)fastmail.fm), Feb 23 2008 %D A005163 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %D A005163 R. P. Stanley, A baker's dozen of conjectures concerning plane partitions, pp. 285-293 of "Combinatoire Enumerative (Montreal 1985)", Lect. Notes Math. 1234, 1986. %H A005163 Christoph Koutschan, <a href="/A005163/b005163.txt">Table of n, a(n) for n = 1..131</a> %H A005163 Roger E. Behrend, Ilse Fischer, and Christoph Koutschan, <a href="https://arxiv.org/abs/2309.08446">Diagonally symmetric alternating sign matrices</a>, arXiv:2309.08446 [math.CO], 2023. %H A005163 Mireille Bousquet-Mélou and Laurent Habsieger, <a href="https://doi.org/10.1016/0012-365X(94)00125-3">Sur les matrices à signes alternants</a>, [On alternating-sign matrices] in Formal power series and algebraic combinatorics (Montreal, PQ, 1992). Discrete Math. 139 (1995), 57-72. %H A005163 D. P. Robbins, <a href="https://arxiv.org/abs/math/0008045">Symmetry classes of alternating sign matrices</a>, arXiv:math/0008045 [math.CO], 2000. %H A005163 R. P. Stanley, <a href="/A005130/a005130.pdf">A baker's dozen of conjectures concerning plane partitions</a>, pp. 285-293 of "Combinatoire Enumerative (Montreal 1985)", Lect. Notes Math. 1234, 1986. Preprint. [Annotated scanned copy] %K A005163 nonn,easy,nice %O A005163 1,2 %A A005163 _N. J. A. Sloane_ and _Simon Plouffe_ %E A005163 More terms (taken from Bousquet-Mélou & Habsieger's paper) from Herman Jamke (hermanjamke(AT)fastmail.fm), Feb 23 2008