login
Numbers k such that k, k+1 and k+2 all have at least three divisors with the same value of the Euler totient function (A000010).
1

%I #7 Sep 01 2024 19:29:12

%S 14052608,83025998,87703714,93978520,117345150,163338174,213589088,

%T 218539880,294321950,369698434,401177798,463425920,470217824,

%U 497434040,529524918,539318438,554556078,559474838,581302358,584754848,608842934,612448640,617445814,625591966

%N Numbers k such that k, k+1 and k+2 all have at least three divisors with the same value of the Euler totient function (A000010).

%C Numbers k such that k, k+1 and k+2 are all in A359565.

%C There must be 3 or more divisors of k that have the same Euler totient value, and ditto for k+1 and k+2, but those values may differ as among k, k+1, and k+2. - _Harvey P. Dale_, Sep 01 2024

%t q[n_] := Max[Tally[EulerPhi[Divisors[n]]][[;; , 2]]] > 2; seq[kmax_] := Module[{s = {}, q1 = 0, q2 = 0, q3}, Do[q3 = q[k]; If[q1 && q2 && q3, AppendTo[s, k-2]]; q1=q2; q2=q3, {k, 3, kmax}]; s]; seq[10^8]

%t SequencePosition[Table[If[Max[Tally[EulerPhi[Divisors[n]]][[;;,2]]]>2,1,0],{n,88*10^6}],{1,1,1}] [[;;,1]] (* The program generates the first 3 terms of the sequence. *) (* _Harvey P. Dale_, Sep 01 2024 *)

%o (PARI) is(k) = vecmax(matreduce(apply(x->eulerphi(x), divisors(k)))[,2]) > 2;

%o lista(kmax) = {my(q1 = 0, q2 = 0, q3); for(k = 3, kmax, q3 = is(k); if(q1 && q2 && q3, print1(k-2, ", ")); q1 = q2; q2 = q3);}

%Y Subsequence of A359565 and A373529.

%Y Cf. A000010, A102190.

%K nonn

%O 1,1

%A _Amiram Eldar_, Jun 08 2024