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 A343261 #18 Apr 11 2021 09:23:23
%S A343261 2,3,14,110,1154,15127,238142,4379769,92198402,2186871698,57721023502,
%T A343261 1678243366813,53301709843202,1836220544383695,68200709735854334,
%U A343261 2716906424134261502,115561578124838522882,5227260815326346060059,250566480717349417632398
%N A343261 a(n) = 2 * T(n,(n+2)/2) where T(n,x) is a Chebyshev polynomial of the first kind.
%H A343261 Seiichi Manyama, <a href="/A343261/b343261.txt">Table of n, a(n) for n = 0..386</a>
%H A343261 Wikipedia, <a href="https://en.wikipedia.org/wiki/Chebyshev_polynomials">Chebyshev polynomials</a>.
%F A343261 a(n) = 2 * cos(n*arccos((n+2)/2)).
%F A343261 a(n) = 2 * n * Sum_{k=0..n} n^k * binomial(n+k,2*k)/(n+k) for n > 0.
%F A343261 a(n) ~ exp(2) * n^n. - _Vaclav Kotesovec_, Apr 09 2021
%t A343261 Table[2*ChebyshevT[n, (n+2)/2], {n, 0, 18}] (* _Amiram Eldar_, Apr 09 2021 *)
%o A343261 (PARI) a(n) = 2*polchebyshev(n, 1, (n+2)/2);
%o A343261 (PARI) a(n) = round(2*cos(n*acos((n+2)/2)));
%o A343261 (PARI) a(n) = if(n==0, 2, 2*n*sum(k=0, n, n^k*binomial(n+k, 2*k)/(n+k)));
%Y A343261 Main diagonal of A299741.
%Y A343261 Cf. A115066, A342167, A342206, A343259, A343260.
%K A343261 nonn
%O A343261 0,1
%A A343261 _Seiichi Manyama_, Apr 09 2021