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 A229875 #13 Jun 19 2025 19:25:57
%S A229875 2,3,5,7,2,2,5,7,1,2,7,2,4,5,7,1,4,5,1,2,5,7,8,7,8,1,4,5,2,7,8,4,8,7,
%T A229875 8,5,8,1,2,2,7,1,4,5,1,2,7,8,1,4,5,8,4,4,5,8,1,4,7,8,1,5,2,5,4,7,4,5,
%U A229875 2,8,7,1,2,1,7,2,7,2,4,8,4,2,2,2,5,4
%N A229875 Iterated sum-of-digits of palindromic prime; or digital root of palindromic prime.
%C A229875 Integers with digital root 3, 6 or 9 are divisible by 3, so 3 is the only palindromic prime with digital root 3 and there are no palindromic primes with digital root 6 or 9.
%H A229875 Shyam Sunder Gupta, <a href="/A229875/b229875.txt">Table of n, a(n) for n = 1..5953</a>
%F A229875 a(n) = A010888(A002385(n)). - _R. J. Mathar_, Sep 09 2015
%e A229875 a(7)=5 because the 7th palindromic prime is 131 and 1+3+1 = 5.
%t A229875 t = {}; Do[z = n*10^(IntegerLength[n] - 1) + FromDigits@Rest@Reverse@IntegerDigits[n]; If[PrimeQ[z], AppendTo[t, Mod[z, 9]]], {n, 1, 99999}]; Insert[t, 2, 5]
%t A229875 Mod[#,9]&/@Select[Prime[Range[9000]],PalindromeQ] (* _Harvey P. Dale_, Mar 25 2025 *)
%Y A229875 Cf. A002385, A010888, A038194.
%K A229875 nonn,base,less
%O A229875 1,1
%A A229875 _Shyam Sunder Gupta_, Oct 02 2013