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A067720 Numbers k such that phi(k^2+1) = k*phi(k+1). 4

%I #18 Nov 21 2020 14:01:32

%S 1,2,4,6,8,10,16,36,40,66,126,130,150,156,180,210,240,250,256,270,280,

%T 306,396,400,420,430,466,490,556,570,576,646,690,700,750,760,826,906,

%U 910,936,946,966,1060,1096,1150,1276,1290,1306,1320,1366,1566,1570

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

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

%C Superset of A070689. Is a(5)=8 the only additional value? - _Ralf Stephan_, Feb 11 2004

%H Amiram Eldar, <a href="/A067720/b067720.txt">Table of n, a(n) for n = 1..10000</a>

%t Select[Range[2000], EulerPhi[#^2 + 1] == #*EulerPhi[# + 1] &] (* _Amiram Eldar_, Nov 21 2020 *)

%o (PARI) isok(k) = eulerphi(k^2+1) == k*eulerphi(k+1); \\ _Michel Marcus_, Nov 21 2020

%Y Cf. A000010, A070689.

%K nonn

%O 1,2

%A _Benoit Cloitre_, Feb 05 2002

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Last modified March 29 00:26 EDT 2024. Contains 371264 sequences. (Running on oeis4.)