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 A007907 #43 Jun 01 2025 08:15:53
%S A007907 1,11,121,1221,12321,123321,1234321,12344321,123454321,1234554321,
%T A007907 12345654321,123456654321,1234567654321,12345677654321,
%U A007907 123456787654321,1234567887654321,12345678987654321
%N A007907 Concatenation of sequence (1, 2, ..., floor((n-1)/2), floor(n/2), floor(n/2)-1, ..., 1) for n >= 1.
%C A007907 For n < 4900, a(2) = A259937(1) = 11, a(19) = A173426(10) = 12345678910987654321, a(20) = A259937(10) = 1234567891010987654321 and a(4891) = A173426(2446) = 1234567..244524462445..7654321 are primes (see A173426 and A259937). - _XU Pingya_, May 19 2017
%D A007907 M. Le, The Primes in the Smarandache Symmetric Sequences, Smarandache Notions Journal, Vol. 10, No. 1-2-3, 1999, 174-175.
%H A007907 Michael De Vlieger, <a href="/A007907/b007907.txt">Table of n, a(n) for n = 1..405</a> [The list of terms has an interesting visual appearance]
%H A007907 F. Smarandache, <a href="http://www.gallup.unm.edu/~smarandache/OPNS.pdf">Only Problems, Not Solutions!</a>
%H A007907 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SmarandacheSequences.html">Smarandache Sequences</a>
%t A007907 Table[FromDigits@ Flatten@ Map[IntegerDigits, Apply[Join, {#, If[OddQ@ n, Rest@ #, #] &@ Reverse@ #}]] &@ Range@ Ceiling[n/2], {n, 17}] (* _Michael De Vlieger_, May 20 2017 *)
%Y A007907 Cf. A173426, A259937.
%K A007907 nonn,base
%O A007907 1,2
%A A007907 R. Muller