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 A061981 #42 Dec 29 2024 14:27:26
%S A061981 0,0,4,20,72,232,716,2172,6544,19664,59028,177124,531416,1594296,
%T A061981 4782940,14348876,43046688,129140128,387420452,1162261428,3486784360,
%U A061981 10460353160,31381059564,94143178780,282429536432,847288609392
%N A061981 a(n) = 3^n - 2*n - 1.
%H A061981 Harry J. Smith, <a href="/A061981/b061981.txt">Table of n, a(n) for n = 0..200</a>
%H A061981 Brandon Alberts and Jack Klys, <a href="https://arxiv.org/abs/1611.05595">The distribution of H8-extensions of quadratic fields</a>, arXiv:1611.05595 [math.NT], 2016. See p. 17.
%H A061981 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5,-7,3).
%F A061981 From _Bruno Berselli_, Jan 31 2012: (Start)
%F A061981 G.f.: 4*x^2/((1-3*x)*(1-x)^2).
%F A061981 a(n) = A186948(n) - 1.
%F A061981 a(n+2) = 4*A000340(n). (End)
%F A061981 From _Deisy J. Camacho_, Feb 26 2021 (Start)
%F A061981 a(n) = Sum_{j=2..n} Sum_{i=0..j} n!/((j-i)!*i!*(n-j)!).
%F A061981 a(n) = 4 + 4*a(n-1) - 3*a(n-2). (End)
%F A061981 E.g.f.: exp(3*x) - (2*x + 1)*exp(x). - _G. C. Greubel_, Jun 13 2022
%t A061981 Table[3^n-2n-1,{n,0,30}] (* or *) LinearRecurrence[{5,-7,3},{0,0,4},30] (* _Harvey P. Dale_, Mar 30 2018 *)
%o A061981 (PARI) a(n) = { 3^n - 2*n - 1 } \\ _Harry J. Smith_, Jul 29 2009
%o A061981 (SageMath) [3^n -(2*n+1) for n in (0..40)] # _G. C. Greubel_, Jun 13 2022
%Y A061981 Column of A061980.
%Y A061981 Cf. A000340, A186948.
%K A061981 nonn,easy
%O A061981 0,3
%A A061981 _Henry Bottomley_, May 24 2001