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%I #17 Aug 18 2019 13:52:54
%S 271,281,21491,21991,22091,22481,23081,23971,24071,25951,26681,26981,
%T 27271,27431,27691,27791,28031,28661,28921,28961,29021,29191,29251,
%U 29411,29671,2129891,2131991,2141791,2141891,2151791,2157091,2161591,2179391,2191291
%N Primes such that applying "reverse and add" twice produces two more primes
%C Some observations:
%C 1. For all terms, the first digit is 2, last digit is 1, number of digits is odd: 3,5,7,...
%C 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 3. Applying "reverse and add" a third time always produces composites. - _Zak Seidov_, Dec 09 2013
%e 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 Transpose[Select[Table[{Prime[i],And@@PrimeQ/@NestList[#+FromDigits[ Reverse[ IntegerDigits[#]]]&,Prime[i],2]},{i,500000}],#[[2]] == True&]][[1]]
%Y Cf. A061783.
%K nonn,base
%O 1,1
%A _Harvey P. Dale_, Nov 27 2010
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