The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 (list; graph; refs; listen; history; text; internal format)
 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 LINKS 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 Adjacent sequences:  A174399 A174400 A174401 * A174403 A174404 A174405 KEYWORD nonn,base AUTHOR Harvey P. Dale, Nov 27 2010 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified August 5 08:27 EDT 2021. Contains 346464 sequences. (Running on oeis4.)