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 A122631 a(n+1) = gpf(2*prime(a(n-1)) + prime(a(n))) where gpf = greatest prime factor, with a(0)=1, a(1)=2. 1
 1, 2, 7, 23, 13, 23, 11, 197, 421, 113, 13, 17, 47, 47, 211, 191, 1249, 499, 1039, 79, 97, 23, 367, 881, 337, 409, 2447, 277, 857, 487, 61, 83, 997, 53, 16007, 1033, 43, 16657, 101, 67, 19, 3, 139, 269, 107, 109, 197, 2399, 7927, 41257, 47, 103, 197, 179, 11, 719, 5503 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS This sequence is eventually cyclic with period 360. LINKS Reinhard Zumkeller and Charles R Greathouse IV, Table of n, a(n) for n = 0..10000 (terms below 97 from Zumkeller) EXAMPLE 2*prime(7) + prime(23) = 2*17 + 83 = 117 = 3*3*13, so a(4) = gpf(117) = 13. MATHEMATICA Transpose[NestList[{#[[2]], FactorInteger[2Prime[ #[[1]]] +Prime[ #[[2]]]][[ -1, 1]]} &, {1, 2}, 900]][[2]] PROG (Haskell) a122631 n = a122631_list !! (n-1) a122631_list = 1 : 2 : map a006530 (zipWith (+) (map ((2 *) . a000040) a122631_list) (map a000040 (tail a122631_list))) -- Reinhard Zumkeller, Aug 07 2011 (PARI) a=[1, 2]; for(n=2, 50, my(f=factor(2*prime(a[n-1])+prime(a[n]))[, 1]); a=concat(a, f[#f])); a \\ Charles R Greathouse IV, Jun 19 2013 CROSSREFS Cf. A006530. Sequence in context: A153551 A125285 A170870 * A269313 A147969 A235985 Adjacent sequences: A122628 A122629 A122630 * A122632 A122633 A122634 KEYWORD nice,nonn,easy AUTHOR Carlos Alves, Oct 29 2006 EXTENSIONS Offset fixed and initial 1 prepended by Reinhard Zumkeller, Aug 07 2011 STATUS approved

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Last modified September 8 17:50 EDT 2024. Contains 375753 sequences. (Running on oeis4.)