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 A361272 #21 Mar 09 2023 17:33:46 %S A361272 1,1,1,3,6,12,20,32,47,67,91,121,156,198,246,302,365,437,517,607,706, %T A361272 816,936,1068,1211,1367,1535,1717,1912,2122,2346,2586,2841,3113,3401, %U A361272 3707,4030,4372,4732,5112,5511,5931,6371,6833,7316,7822,8350,8902,9477,10077 %N A361272 Number of 1243-avoiding even Grassmannian permutations of size n. %C A361272 A permutation is said to be Grassmannian if it has at most one descent. A permutation is even if it has an even number of inversions. %C A361272 a(n) is also the number of sigma-avoiding even Grassmannian permutations of size n, where sigma is any of the patterns 2134, 2341, or 4123. %H A361272 Juan B. Gil and Jessica A. Tomasko, <a href="https://arxiv.org/abs/2207.12617">Pattern-avoiding even and odd Grassmannian permutations</a>, arXiv:2207.12617 [math.CO], 2022. %H A361272 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (3,-2,-2,3,-1). %F A361272 G.f.: -(2*x^4-4*x^3+2*x-1)/((x+1)*(x-1)^4). %F A361272 a(n) = (57 - 9*(-1)^n - 28*n + 6*n^2 + 4*n^3)/48. - _Stefano Spezia_, Mar 09 2023 %e A361272 For n=4 the a(4) = 6 permutations are 1234, 1342, 1423, 2314, 3124, 3412. %Y A361272 Cf. A175287, A356185, A361273. %K A361272 nonn,easy %O A361272 0,4 %A A361272 _Juan B. Gil_, Mar 09 2023