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 A361271 #21 Mar 10 2023 11:04:28 %S A361271 0,0,1,2,6,9,19,25,44,54,85,100,146,167,231,259,344,380,489,534,670, %T A361271 725,891,957,1156,1234,1469,1560,1834,1939,2255,2375,2736,2872,3281, %U A361271 3434,3894,4065,4579,4769,5340,5550,6181,6412,7106,7359,8119,8395,9224,9524,10425 %N A361271 Number of 1342-avoiding odd Grassmannian permutations of size n. %C A361271 A permutation is said to be Grassmannian if it has at most one descent. A permutation is odd if it has an odd number of inversions. %C A361271 a(n) is also the number of 3124-avoiding odd Grassmannian permutations of size n. %H A361271 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 A361271 <a href="/index/Rec#order_07">Index entries for linear recurrences with constant coefficients</a>, signature (1,3,-3,-3,3,1,-1). %F A361271 G.f.: x^2*(x^4+x^2+x+1)/((1+x)^3*(1-x)^4). %e A361271 For n=4 the a(4)=6 permutations are 1243, 1324, 2134, 2341, 2413, 4123. %o A361271 (PARI) seq(n) = Vec(x^2*(x^4+x^2+x+1)/((1+x)^3*(1-x)^4) + O(x*x^n), -n-1) \\ _Andrew Howroyd_, Mar 07 2023 %Y A361271 Cf. A356185, A361270, A361274. %K A361271 nonn,easy %O A361271 0,4 %A A361271 _Juan B. Gil_, Mar 07 2023