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A174402
Primes such that applying "reverse and add" twice produces two more primes
0
271, 281, 21491, 21991, 22091, 22481, 23081, 23971, 24071, 25951, 26681, 26981, 27271, 27431, 27691, 27791, 28031, 28661, 28921, 28961, 29021, 29191, 29251, 29411, 29671, 2129891, 2131991, 2141791, 2141891, 2151791, 2157091, 2161591, 2179391, 2191291
OFFSET
1,1
COMMENTS
Some observations:
1. For all terms, the first digit is 2, last digit is 1, number of digits is odd: 3,5,7,...
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.
3. Applying "reverse and add" a third time always produces composites. - Zak Seidov, Dec 09 2013
EXAMPLE
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.
MATHEMATICA
Transpose[Select[Table[{Prime[i], And@@PrimeQ/@NestList[#+FromDigits[ Reverse[ IntegerDigits[#]]]&, Prime[i], 2]}, {i, 500000}], #[[2]] == True&]][[1]]
CROSSREFS
Cf. A061783.
Sequence in context: A290643 A104844 A086003 * A048295 A257210 A020363
KEYWORD
nonn,base
AUTHOR
Harvey P. Dale, Nov 27 2010
STATUS
approved