Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #28 Aug 03 2014 14:01:41
%S 7,9,15,17,21,23,25,27,31,33,35,39,41,43,45,47,49,51,55,57,63,65,69,
%T 71,73,75,77,79,81,85,87,89,91,93,95,97,99,103,105,109,111,113,115,
%U 117,119,121,123,125,127,129,133,135,137,141,143,145,147,151,153,155,157,159,161,165,167,169,171,175,177,183,185,187,189,191,193,195,199
%N Odd values of n such that the polynomial 1+x+x^2+...+x^(n-1) is reducible over GF(2).
%C This sequence is the union of the odd composite numbers and the primes for which 2 is not a primitive root.
%H Vincenzo Librandi, <a href="/A217460/b217460.txt">Table of n, a(n) for n = 1..1000</a>
%t nn = 200; Union[Select[Range[3, nn, 2], ! PrimeQ[#] &], Select[Prime[Range[2, PrimePi[nn]]], PrimitiveRoot[#] =!= 2 &]] (* _T. D. Noe_, Sep 19 2012 *)
%o (PARI) for(i=4, 200, if(isprime(i), if(znorder(Mod(2,i))!=(i-1), print(i)), if(i%2==1, print(i))))
%o (PARI) for(i=0, 200, i++; if(matsize(factormod((x^i+1)/(x+1), 2, 1))[1]>1, print(i)))
%Y Cf. A002326, A001122, A216838.
%K nonn
%O 1,1
%A _V. Raman_, Oct 04 2012