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A174402
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Primes such that applying "reverse and add" twice produces two more primes
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0
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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
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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EXAMPLE
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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.
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MATHEMATICA
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Transpose[Select[Table[{Prime[i], And@@PrimeQ/@NestList[#+FromDigits[ Reverse[ IntegerDigits[#]]]&, Prime[i], 2]}, {i, 500000}], #[[2]] == True&]][[1]]
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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