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).

1, 11, 131, 1331, 13531, 135531, 1357531, 13577531, 135797531, 1357997531, 135791197531, 13579111197531, 1357911131197531, 135791113131197531, 13579111315131197531, 1357911131515131197531, 135791113151715131197531, 13579111315171715131197531

Offset: 1

Author

XU Pingya, May 22 2017

Keywords

• base
• easy
• nonn

Comments

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

Links

• F. Smarandache, Sequences of Numbers Involved in Unsolved Problems.
• Eric Weisstein's World of Mathematics, Smarandache Sequences

Crossrefs

Cf. A007907.

Programs

Mathematica

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 *)