%I
%S 8,12,15,16,20,21,24,28,30,32,33,35,36,39,40,42,44,45,48,51,52,55,56,
%T 57,60,63,64,65,66,68,69,70,72,75,76,77,78,80,84,85,87,88,90,91,92,93,
%U 95,96,99,100,102,104,105,108,110,111,112,114,115,116,117,119,120,123
%N Positive integers that do not have a primitive root.
%C Numbers n such that the cyclotomic polynomial Phi(n,x) is reducible over Zp for all primes p. Harrison shows that this is equivalent to n>2 and the discriminant of Phi(n,x), A004124(n), being a square.  _T. D. Noe_, Nov 06 2007
%C The multiplicative group modulo n is noncyclic. See the complement A033948.  _Wolfdieter Lang_, Mar 14 2012. See A281854 for the groups.  _Wolfdieter Lang_, Feb 04 2017
%C Numbers n with the property that there exists a natural number m with 1<m<n1 and m^2 == 1 mod n.  _Reinhard Muehlfeld_, May 27 2014
%C Also, numbers n for which A000010(n)>A002322(n), or equivalently A034380(n)>1.  _Ivan Neretin_, Mar 28 2015
%D I. Niven and H. S. Zuckerman, An Introduction to the Theory of Numbers, 4th edition, page 62, Theorem 2.25.
%H T. D. Noe, <a href="/A033949/b033949.txt">Table of n, a(n) for n = 1..10000</a>
%H Brett A. Harrison, <a href="http://www.jstor.org/stable/27642336">On the reducibility of cyclotomic polynomials over finite fields</a>, Amer. Math. Monthly, Vol 114, No. 9 (2007), 813818
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Primitive_root_modulo_n">Primitive root modulo n</a>
%F Positive integers except 1, 2, 4 and numbers of the form p^i and 2p^i, where p is an odd prime and i >= 1.
%p m := proc(n) local k, r; r := 1; if n = 2 then return false fi;
%p for k from 1 to n do if igcd(n,k) = 1 then r := modp(r*k,n) fi od; r end:
%p select(n > m(n) = 1, [$1..123]); # _Peter Luschny_, May 25 2017
%t Select[Range[2,130],!IntegerQ[PrimitiveRoot[#]]&] (* _Harvey P. Dale_, Oct 25 2011 *)
%t a[n_] := Module[{j, l = {}}, While[Length[l]<n, For[j = 1+If[l=={}, 0, l // Last], True, j++, If[EulerPhi[j] > CarmichaelLambda[j], AppendTo[l, j]; Break[]]]]; l[[n]]]; Array[a, 100] (* _JeanFrançois Alcover_, May 29 2018, after _Alois P. Heinz_'s Maple code for A277915 *)
%o (Sage) print [n for n in range(1,100) if not Integers(n).multiplicative_group_is_cyclic()] # _Ralf Stephan_, Mar 30 2014
%o (Haskell)
%o a033949 n = a033949_list !! (n1)
%o a033949_list = filter
%o (\x > any ((== 1) . (`mod` x) . (^ 2)) [2 .. x2]) [1..]
%o  _Reinhard Zumkeller_, Dec 10 2014
%o (PARI) is(n)=n>7 && (!isprimepower(if(n%2,n,n/2))  n>>valuation(n,2)==1) \\ _Charles R Greathouse IV_, Oct 08 2016
%Y Cf. A033948, A193305 (composites with primitive root).
%Y Column k=1 of A277915, A281854.
%K nonn
%O 1,1
%A Calculated by _Jud McCranie_
