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 A287294 #18 Feb 16 2025 08:33:46
%S A287294 1,313,53135,7531357,975313579,1197531357911,13119753135791113,
%T A287294 151311975313579111315,1715131197531357911131517,
%U A287294 19171513119753135791113151719,211917151311975313579111315171921,2321191715131197531357911131517192123
%N A287294 Concatenation of sequence (2n-1,2n-3,..,3,1,3,..,2n-3,2n-1).
%C A287294 For n <= 1400, a(2) = 313, a(10) = 19171513119753135791113151719, a(12) = 2321191715131197531357911131517192123 and a(110) = 219217..313..217219 are primes.
%H A287294 F. Smarandache, <a href="http://www.gallup.unm.edu/~smarandache/Sequences-book.pdf">Sequences of Numbers Involved in Unsolved Problems</a>.
%H A287294 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SmarandacheSequences.html">Smarandache Sequences</a>
%t A287294 Block[{nn = 12, s}, s = IntegerDigits@ Range[1, 2 nn - 1, 2]; Table[FromDigits@ Flatten@ Join[Reverse[Rest@ #], #] &@ Take[s, n], {n, nn}]] (* _Michael De Vlieger_, May 23 2017 *)
%Y A287294 Cf. A007942.
%K A287294 base,easy,nonn
%O A287294 1,2
%A A287294 _XU Pingya_, May 22 2017