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A039652
Becomes prime after n iterations of f(x) = phi(x)+1 (least inverse of A039651).
10
2, 1, 15, 35, 69, 255, 535, 949, 1957, 2513, 2923, 4531, 17701, 22957, 54589, 79421, 80029, 84493, 98581, 102827, 115243, 239111, 291149, 310813, 362621, 398893, 598341, 801923, 838307, 1063493, 1079833, 1123813, 1311121, 1329403, 1582439
OFFSET
0,1
COMMENTS
Of the terms n <= 66, all are semiprimes except those for n = 0, 1, 5, and 19. Why? - T. D. Noe, Oct 17 2013
MATHEMATICA
nn = 34; t = Table[0, {nn}]; found = 0; n = 0; While[found < nn, n++; len = Length[NestWhileList[EulerPhi[#] + 1 &, n, UnsameQ, All]] - 2; If[len <= nn && t[[len]] == 0, t[[len]] = n; found++]]; t = Join[{2}, t] (* T. D. Noe, Oct 17 2013 *)
KEYWORD
nonn,hard
STATUS
approved