A217460 Odd values of n such that the polynomial 1+x+x^2+...+x^(n-1) is reducible over GF(2).

7, 9, 15, 17, 21, 23, 25, 27, 31, 33, 35, 39, 41, 43, 45, 47, 49, 51, 55, 57, 63, 65, 69, 71, 73, 75, 77, 79, 81, 85, 87, 89, 91, 93, 95, 97, 99, 103, 105, 109, 111, 113, 115, 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

Offset: 1

Author

V. Raman, Oct 04 2012

Keywords

• nonn

Comments

This sequence is the union of the odd composite numbers and the primes for which 2 is not a primitive root.

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A002326, A001122, A216838.

Programs

Mathematica

nn = 200; Union[Select[Range[3, nn, 2], ! PrimeQ[#] &], Select[Prime[Range[2, PrimePi[nn]]], PrimitiveRoot[#] =!= 2 &]] (* T. D. Noe, Sep 19 2012 *)

PARI

for(i=4, 200, if(isprime(i), if(znorder(Mod(2,i))!=(i-1), print(i)), if(i%2==1, print(i))))

PARI

for(i=0, 200, i++; if(matsize(factormod((x^i+1)/(x+1), 2, 1))[1]>1, print(i)))