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 A062373 Ratio of totient to Carmichael's lambda function is 2. 14
 8, 12, 15, 16, 20, 21, 28, 30, 32, 33, 35, 36, 39, 42, 44, 45, 51, 52, 55, 57, 64, 66, 68, 69, 70, 75, 76, 77, 78, 87, 90, 92, 93, 95, 99, 100, 102, 108, 110, 111, 114, 115, 116, 119, 123, 124, 128, 129, 135, 138, 141, 143, 147, 148, 150, 153, 154, 155, 159, 161 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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 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 LINKS R. J. Mathar, Table of n, a(n) for n = 1..20000 FORMULA Solutions to phi(k)/lambda(k) = 2. EXAMPLE From Jianing Song, Jul 28 2018: (Start) (Z/8Z)* = C_2 X C_2, so 8 is a term. (Z/21Z)* = C_2 X C_6, so 21 is a term. (Z/35Z)* = C_2 X C_12, so 35 is a term. (End) MATHEMATICA Reap[ For[ n = 1, n <= 161, n++, If[ EulerPhi[n] / CarmichaelLambda[n] == 2, Sow[n]]]][[2, 1]] (* Jean-François Alcover, Mar 26 2013 *) Select[Range, EulerPhi[#]/CarmichaelLambda[#]==2&] (* Harvey P. Dale, Jun 27 2018 *) PROG (Haskell) a062373 n = a062373_list !! (n-1) a062373_list = filter ((== 2) . a034380) [1..] -- Reinhard Zumkeller, Sep 02 2014 (PARI) isok(n) = eulerphi(n)/lcm(znstar(n)) == 2; \\ Michel Marcus, Jul 28 2018 CROSSREFS Cf. A000010, A002322, A034380, A033948, A062374, A062375, A062376, A062377. Sequence in context: A033949 A175594 A272592 * A180690 A194592 A175132 Adjacent sequences:  A062370 A062371 A062372 * A062374 A062375 A062376 KEYWORD easy,nonn AUTHOR Vladeta Jovovic, Jun 17 2001 EXTENSIONS More terms from Reiner Martin (reinermartin(AT)hotmail.com), Dec 22 2001 STATUS approved

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Last modified July 11 10:19 EDT 2020. Contains 335626 sequences. (Running on oeis4.)