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

0,1

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

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

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

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

Sequence in context: A326600 A272304 A266521 * A374264 A012894 A013076

Adjacent sequences: A039649 A039650 A039651 * A039653 A039654 A039655

Keyword

nonn,hard

Author

David W. Wilson

Status

approved