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%I #15 Jul 13 2019 05:32:10
%S 1,3,5,13,15,35,37,61,73,104,119,157,164,193,194,255,277,313,397,421,
%T 455,457,495,527,541,545,584,613,629,661,665,673,733,757,877,975,997,
%U 1085,1093,1153,1201,1213,1237,1295,1321,1381,1453,1469,1621,1657,1753
%N Numbers k such that phi(k+1) divides phi(k), where phi is A000010.
%C The intersection of this sequence and A057919 is A001274. - _Michel Marcus_, Sep 14 2015
%H Amiram Eldar, <a href="/A057920/b057920.txt">Table of n, a(n) for n = 1..10000</a>
%e 13 is included because phi(14) = 6 divides phi(13) = 12.
%p for n from 1 to 2000 do
%p if modp(numtheory[phi](n),numtheory[phi](n+1)) =0 then
%p printf("%d,",n) ;
%p end if;
%p end do: # _R. J. Mathar_, Sep 14 2015
%t Select[Range[1800], Divisible[EulerPhi[#], EulerPhi[# + 1]] &] (* _Amiram Eldar_, Jul 13 2019 *)
%o (PARI) lista(nn) = for (n=1, nn, if (eulerphi(n) % eulerphi(n+1) == 0, print1(n, ", "))); \\ _Michel Marcus_, Sep 14 2015
%Y Cf. A001274, A057919.
%K nonn
%O 1,2
%A _Leroy Quet_, Nov 11 2000
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