A287295 - OEIS

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 #14 Feb 16 2025 08:33:46

%S 1,11,131,1331,13531,135531,1357531,13577531,135797531,1357997531,

%T 135791197531,13579111197531,1357911131197531,135791113131197531,

%U 13579111315131197531,1357911131515131197531,135791113151715131197531,13579111315171715131197531

%N 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 For n = {2, 3, 13, 16, 19, 21, 67}, a(n) is prime (when n <= 2800).

%H F. Smarandache, <a href="http://www.gallup.unm.edu/~smarandache/Sequences-book.pdf">Sequences of Numbers Involved in Unsolved Problems</a>.

%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SmarandacheSequences.html">Smarandache Sequences</a>

%t 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 Cf. A007907.

%K base,easy,nonn,changed

%O 1,2

%A _XU Pingya_, May 22 2017