OFFSET
1,2
COMMENTS
Prime terms are 11, 19, 23, 31, 41, 47, 59, 67, 71, 79, 89, 101, 109, 127, 131, ...
Up to 10^9, twin prime pairs in this sequence are (137, 139), (461, 463), (1019, 1021), (1427, 1429), (2969, 2971), (4229, 4231).
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
11 is a term because phi(psi(phi(11))) = psi(phi(psi(11))).
MATHEMATICA
psi[n_] := If[n < 1, 0, n Sum[MoebiusMu[d]^2/d, {d, Divisors@n}]]; fQ[n_] := EulerPhi[psi[EulerPhi[n]]] == psi[EulerPhi[psi[n]]]; Select[Range@635, fQ] (* Robert G. Wilson v, Sep 23 2017 *)
PROG
(PARI) a001615(n) = my(f=factor(n)); prod(i=1, #f~, f[i, 1]^f[i, 2] + f[i, 1]^(f[i, 2]-1));
isok(n) = a001615(eulerphi(a001615(n)))==eulerphi(a001615(eulerphi(n))); \\ after Charles R Greathouse IV at A001615
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Altug Alkan, Sep 04 2017
STATUS
approved