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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 (list; graph; refs; listen; history; text; internal format)
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
LINKS
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 *)
CROSSREFS
Sequence in context: A326600 A272304 A266521 * A012894 A013076 A012890
KEYWORD
nonn,hard
AUTHOR
STATUS
approved

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Last modified March 19 01:57 EDT 2024. Contains 370952 sequences. (Running on oeis4.)