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%I #24 May 28 2024 06:47:44
%S 4,6,8,10,12,15,28,66,91,561,946,6601,8911,6840001
%N Composite numbers k such that phi(k) divides 4*(k-1).
%C From _Amiram Eldar_, May 28 2024: (Start)
%C The terms 561, 6601, 8911 and 6840001 are Carmichael numbers (A002997). The next Carmichael number in this sequence is 16187797671051601, and there are no more Carmichael numbers in this sequence below 10^22.
%C a(15) > 10^10. (End)
%t t={}; Do[If[PrimeQ[n]==False && IntegerQ[(n-1)*4/EulerPhi[n]], Print[n]; AppendTo[t, n]], {n,2,10^5}]; t
%t Select[Range[7*10^6],CompositeQ[#]&&Divisible[4(#-1),EulerPhi[#]]&] (* _Harvey P. Dale_, Jul 07 2016 *)
%o (PARI) p=1;forprime(q=2,1e9,for(n=p+1,q-1,if((4*n-4)%eulerphi(n)==0,print1(n", ")));p=q) \\ _Charles R Greathouse IV_, Jul 31 2011
%Y Cf. A000010, A002997.
%K nonn,more
%O 1,1
%A _José María Grau Ribas_, Jul 20 2011
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