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A062373 Ratio of totient to Carmichael's lambda function is 2.

%I A062373 #28 Oct 30 2022 18:19:59
%S A062373 8,12,15,16,20,21,28,30,32,33,35,36,39,42,44,45,51,52,55,57,64,66,68,
%T A062373 69,70,75,76,77,78,87,90,92,93,95,99,100,102,108,110,111,114,115,116,
%U A062373 119,123,124,128,129,135,138,141,143,147,148,150,153,154,155,159,161
%N A062373 Ratio of totient to Carmichael's lambda function is 2.
%C A062373 Numbers k such that the highest order of elements in (Z/kZ)* is phi(n)/2, (Z/kZ)* = the multiplicative group of integers modulo k. Also numbers k such that (Z/kZ)* = C_2 X C_(2r). - _Jianing Song_, Jul 28 2018
%C A062373 Contains the powers of 2 greater than 4, 4 times primes, and semiprimes pq where (p-1)/2 and (q-1)/2 are coprime. If n is odd and in this sequence then so is 2n. - _Charlie Neder_, May 27 2019
%H A062373 R. J. Mathar, <a href="/A062373/b062373.txt">Table of n, a(n) for n = 1..20000</a>
%F A062373 Solutions to phi(k)/lambda(k) = 2.
%e A062373 From _Jianing Song_, Jul 28 2018: (Start)
%e A062373 (Z/8Z)* = C_2 X C_2, so 8 is a term.
%e A062373 (Z/21Z)* = C_2 X C_6, so 21 is a term.
%e A062373 (Z/35Z)* = C_2 X C_12, so 35 is a term. (End)
%t A062373 Reap[ For[ n = 1, n <= 161, n++, If[ EulerPhi[n] / CarmichaelLambda[n] == 2, Sow[n]]]][[2, 1]] (* _Jean-François Alcover_, Mar 26 2013 *)
%t A062373 Select[Range[200],EulerPhi[#]/CarmichaelLambda[#]==2&] (* _Harvey P. Dale_, Jun 27 2018 *)
%o A062373 (Haskell)
%o A062373 a062373 n = a062373_list !! (n-1)
%o A062373 a062373_list = filter ((== 2) . a034380) [1..]
%o A062373 -- _Reinhard Zumkeller_, Sep 02 2014
%o A062373 (PARI) isok(n) = eulerphi(n)/lcm(znstar(n)[2]) == 2; \\ _Michel Marcus_, Jul 28 2018
%Y A062373 Cf. A000010, A002322, A034380, A033948, A062374, A062375, A062376, A062377.
%K A062373 easy,nonn
%O A062373 1,1
%A A062373 _Vladeta Jovovic_, Jun 17 2001
%E A062373 More terms from _Reiner Martin_, Dec 22 2001