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[200], 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]) == 2; \\ Michel Marcus, Jul 28 2018

CROSSREFS

KEYWORD

easy,nonn

AUTHOR

Vladeta Jovovic, Jun 17 2001

EXTENSIONS

More terms from Reiner Martin, Dec 22 2001

STATUS

approved