A039652 Becomes prime after n iterations of f(x) = phi(x)+1 (least inverse of A039651).

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

Author

David W. Wilson

Keywords

• nonn
• hard

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

• T. D. Noe, Table of n, a(n) for n = 0..66

Crossrefs

Cf. A039649, A039650, A039651, A039652, A039653, A039654, A039655, A039656.

Programs

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 *)