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A287295 a(2n-1) is concatenation of sequence (1,3,..,2n-3,2n-1,2n-3,..3,1) and a(2n) is concatenation of sequence (1,3,..,2n-3,2n-1,2n-1,2n-3,..3,1).

%I A287295 #14 Feb 16 2025 08:33:46
%S A287295 1,11,131,1331,13531,135531,1357531,13577531,135797531,1357997531,
%T A287295 135791197531,13579111197531,1357911131197531,135791113131197531,
%U A287295 13579111315131197531,1357911131515131197531,135791113151715131197531,13579111315171715131197531
%N A287295 a(2n-1) is concatenation of sequence (1,3,..,2n-3,2n-1,2n-3,..3,1) and a(2n) is concatenation of sequence (1,3,..,2n-3,2n-1,2n-1,2n-3,..3,1).
%C A287295 For n = {2, 3, 13, 16, 19, 21, 67}, a(n) is prime (when n <= 2800).
%H A287295 F. Smarandache, <a href="http://www.gallup.unm.edu/~smarandache/Sequences-book.pdf">Sequences of Numbers Involved in Unsolved Problems</a>.
%H A287295 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SmarandacheSequences.html">Smarandache Sequences</a>
%t A287295 Block[{nn = 18, s}, s = IntegerDigits@ Range[1, nn, 2]; Table[FromDigits@ Flatten@ Join[#, Reverse[If[EvenQ@ n, #, Most@ #] &@ #]] &@ Take[s, Ceiling[n/2]], {n, nn}]] (* _Michael De Vlieger_, May 23 2017 *)
%Y A287295 Cf. A007907.
%K A287295 base,easy,nonn
%O A287295 1,2
%A A287295 _XU Pingya_, May 22 2017