A067720 Numbers k such that phi(k^2+1) = k*phi(k+1).

1, 2, 4, 6, 8, 10, 16, 36, 40, 66, 126, 130, 150, 156, 180, 210, 240, 250, 256, 270, 280, 306, 396, 400, 420, 430, 466, 490, 556, 570, 576, 646, 690, 700, 750, 760, 826, 906, 910, 936, 946, 966, 1060, 1096, 1150, 1276, 1290, 1306, 1320, 1366, 1566, 1570

Offset: 1

Author

Benoit Cloitre, Feb 05 2002

Keywords

• nonn

Comments

a(n)+1 is prime except for a(5)=8.

Superset of A070689. Is a(5)=8 the only additional value? - Ralf Stephan, Feb 11 2004

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010, A070689.

Programs

Mathematica

Select[Range[2000], EulerPhi[#^2 + 1] == #*EulerPhi[# + 1] &] (* Amiram Eldar, Nov 21 2020 *)

PARI

isok(k) = eulerphi(k^2+1) == k*eulerphi(k+1); \\ Michel Marcus, Nov 21 2020