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
KEYWORD
nonn,base
AUTHOR
Harvey P. Dale, Nov 27 2010
STATUS
approved