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A067720
Numbers k such that phi(k^2+1) = k*phi(k+1).
4
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
1,2
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
MATHEMATICA
Select[Range[2000], EulerPhi[#^2 + 1] == #*EulerPhi[# + 1] &] (* Amiram Eldar, Nov 21 2020 *)
PROG
(PARI) isok(k) = eulerphi(k^2+1) == k*eulerphi(k+1); \\ Michel Marcus, Nov 21 2020
CROSSREFS
Sequence in context: A227992 A175299 A088008 * A078106 A353764 A111082
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Feb 05 2002
STATUS
approved