A043343 - OEIS

%I #22 Nov 13 2024 08:30:20

%S 2,6,12,18,22,30,42,46,54,58,60,62,66,90,94,98,106,118,120,126,134,

%T 138,142,150,158,162,166,174,198,206,210,214,240,242,250,254,262,270,

%U 274,276,278,282,294,298,302,318,330,334,346,348,354,358,378,382,394,398

%N Numbers m such that there is no k > m such that phi(k) = phi(m), where phi is Euler's totient function.

%H Amiram Eldar, <a href="/A043343/b043343.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..980 from Paul Tek)

%H Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).

%F A066659(a(n)) = 0.

%F A028476(a(n)) = a(n).

%t Flatten@ Position[#, 0] &@ Table[k = n + 1; While[And[k <= 2 n + 1, EulerPhi@ k != EulerPhi@ n], k++]; Boole[k < 2 n + 1] k, {n, 400}] (* _Michael De Vlieger_, Dec 31 2016 *)

%o (PARI) is(k) = #select(x -> x>k, invphi(eulerphi(k))) == 0; \\ _Amiram Eldar_, Nov 12 2024, using _Max Alekseyev_'s invphi.gp

%Y Cf. A000010, A028476, A066659.

%K nonn

%O 1,1

%A _Vladeta Jovovic_, Jan 12 2002

%E Offset corrected by _Paul Tek_, Sep 25 2015

%E Definition clarified by _Alonso del Arte_, Dec 31 2016