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 A308520 #8 Jun 11 2022 20:16:49
%S A308520 1,3,9,27,84,287,1116,4984,25368,145277,924684,6475018,49464756,
%T A308520 409371731,3648595216,34841512504,354892721168,3840839273849,
%U A308520 44012775982132,532368664987942,6778328366073724,90619575089479631,1269184691838666152,18583725601041230532
%N A308520 Expansion of e.g.f. exp(x)*(1 + x + x^2/2)*(sec(x) + tan(x)).
%C A308520 Boustrophedon transform of A000124 (central polygonal numbers).
%H A308520 <a href="/index/Bo#boustrophedon">Index entries for sequences related to boustrophedon transform</a>
%t A308520 nmax = 23; CoefficientList[Series[Exp[x] (1 + x + x^2/2) (Sec[x] + Tan[x]), {x, 0, nmax}], x] Range[0, nmax]!
%t A308520 t[n_, 0] := n (n + 1)/2 + 1; t[n_, k_] := t[n, k] = t[n, k - 1] + t[n - 1, n - k]; a[n_] := t[n, n]; Array[a, 24, 0]
%o A308520 (Python)
%o A308520 from itertools import count, islice, accumulate
%o A308520 def A308520_gen(): # generator of terms
%o A308520 blist = tuple()
%o A308520 for i in count(0):
%o A308520 yield (blist := tuple(accumulate(reversed(blist),initial=i*(i+1)//2+1)))[-1]
%o A308520 A308520_list = list(islice(A308520_gen(),30)) # _Chai Wah Wu_, Jun 11 2022
%Y A308520 Cf. A000111, A000124, A000718, A000746.
%K A308520 nonn
%O A308520 0,2
%A A308520 _Ilya Gutkovskiy_, Jun 04 2019