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 A174402 #30 Jul 30 2025 00:58:14
%S A174402 271,281,21491,21991,22091,22481,23081,23971,24071,25951,26681,26981,
%T A174402 27271,27431,27691,27791,28031,28661,28921,28961,29021,29191,29251,
%U A174402 29411,29671,2129891,2131991,2141791,2141891,2151791,2157091,2161591,2179391,2191291
%N A174402 Primes such that applying "reverse and add" twice produces two more primes.
%C A174402 Some observations:
%C A174402 1. For all terms, the first digit is 2, last digit is 1, number of digits is odd: 3,5,7,...
%C A174402 2. The sequence is infinite. Number of 3-digit terms is 2, number of 5-digit terms is 23, number of 7-digit terms is 585, number of 9-digit terms is 26611.
%C A174402 3. Applying "reverse and add" a third time always produces composites. - _Zak Seidov_, Dec 09 2013
%H A174402 Harvey P. Dale, <a href="/A174402/b174402.txt">Table of n, a(n) for n = 1..610</a> (All terms with 7 or fewer digits.)
%e A174402 21491 is included because (1) it is prime, and (2) 21491 + 19412 = 40903 which is prime, and (3) 40903 + 30904 = 71807 which also is prime.
%t A174402 Transpose[Select[Table[{Prime[i],And@@PrimeQ/@NestList[#+FromDigits[ Reverse[ IntegerDigits[#]]]&,Prime[i],2]},{i,500000}],#[[2]] == True&]][[1]]
%t A174402 tmpQ[p_]:=AllTrue[Rest[NestList[#+IntegerReverse[#]&,p,2]],PrimeQ]; Select[Prime[Range[163000]],tmpQ] (* _Harvey P. Dale_, Feb 05 2025 *)
%Y A174402 Cf. A061783.
%K A174402 nonn,base
%O A174402 1,1
%A A174402 _Harvey P. Dale_, Nov 27 2010