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 A003138 #24 Nov 06 2022 07:44:28
%S A003138 24,36,56,90,149,252,435,765,1363,2455,4466,8190,15122,28085,52427,
%T A003138 98303,185041,349524,662257,1258290,2396744,4575603,8753329,16777215,
%U A003138 32212254,61946643,119304646,230087533
%N A003138 Nearest integer to 24*(2^n - 1)/n.
%C A003138 In case of a tie, round up. - _N. J. A. Sloane_, Nov 06 2022
%H A003138 G. C. Greubel, <a href="/A003138/b003138.txt">Table of n, a(n) for n = 1..1000</a>
%H A003138 R. J. Penrose, <a href="http://people.maths.ox.ac.uk/lmason/Tn/TN1-25/No10%202%20July%201980.pdf">Puzzle</a>, Twistor Newsletter, No. 10 (July 1980), p. 22.
%H A003138 R. J. Penrose, <a href="/A003138/a003138.pdf">Puzzle</a>, Twistor Newsletter, No. 10 (July 1980), p. 22. [Cached copy]
%H A003138 R. J. Penrose, <a href="http://people.maths.ox.ac.uk/lmason/Tn/41/TN41.pdf">Solution to puzzle</a>, Twistor Newsletter, No. 41, p. 37, 1996.
%H A003138 R. J. Penrose, <a href="/A003138/a003138_1.pdf">Solution to puzzle</a>, Twistor Newsletter, No. 41, p. 37, 1996. [Cached copy]
%H A003138 Z. Sunic, <a href="https://arxiv.org/abs/math/0612080">Tree morphisms, transducers and integer sequences</a>, arXiv:math/0612080 [math.CO], 2006.
%t A003138 A003138[n_]:= With[{m=24*(2^n-1)/n}, If[Floor[m+1/2] -Floor[m]>=1/2, Ceiling[m], Floor[m]]];
%t A003138 Table[A003138[n], {n,70}] (* _G. C. Greubel_, Nov 04 2022 *)
%o A003138 (Magma) [Round(24*(2^n -1)/n): n in [1..70]]; // _G. C. Greubel_, Nov 04 2022
%o A003138 (SageMath) [round(24*(2^n -1)/n) for n in range(1,71)] # _G. C. Greubel_, Nov 04 2022
%o A003138 (PARI) a(n) = round(24*(2^n - 1)/n); \\ _Michel Marcus_, Nov 05 2022
%Y A003138 Cf. A003176, A003177, A121056.
%K A003138 nonn,easy
%O A003138 1,1
%A A003138 _N. J. A. Sloane_