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 A261245 #20 May 13 2019 13:46:43 %S A261245 1,3,9,111,333,999,12321,36963,1001001,3003003,9009009,111111111, %T A261245 333333333,999999999,12333333321,36999999963,1002003002001, %U A261245 3006009006003,111222333222111,333666999666333,12345678987654321 %N A261245 The 21 palindromic divisors of the palindrome 12345678987654321. %C A261245 There are 45 divisors of 12345678987654321. %C A261245 Motivated by A261072 by _Ilya Gutkovskiy_, which has only six palindromic divisors, namely 1, 3, 7, 9, 171, 1234567890987654321. %C A261245 12345678987654321 = A002275(9)^2 = 111111111^2 (observed by _Jon E. Schoenfield_). The repunit R(9) has the eight palindromic divisors 1, 3, 9, 111, 333, 1001001, 3003003, 111111111. %C A261245 All entries a(n) can be obtained by multiplying two palindromic divisors of R(9). %t A261245 Select[Divisors[12345678987654321],PalindromeQ] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, May 13 2019 *) %Y A261245 Cf. A002275, A261072. %K A261245 nonn,easy,base,fini,full %O A261245 1,2 %A A261245 _Wolfdieter Lang_, Aug 22 2015