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A107220 Numbers n so that 1 + (x + x^3 + x^5 + x^7 + ...+ x^(2*n+1)) is irreducible over GF(2). 0
1, 3, 5, 7, 9, 13, 23, 27, 31, 37, 63, 69, 117, 119, 173, 219, 223, 247, 307, 363, 383, 495, 695, 987, 2519, 3919, 4633, 6537, 8881, 12841, 15935, 16383, 16519, 26525, 34415, 95139 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

All terms are odd as irreducible polynomials over GF(2) necessarily have an odd number of nonzero coefficients.

Next term > 10^5. - Joerg Arndt, Apr 28 2012

LINKS

Table of n, a(n) for n=1..36.

Joerg Arndt, Matters Computational (The Fxtbook), section 40.9.10 "Irreducible alternating polynomials", pp.853

N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references)

EXAMPLE

The number 5 is in the sequence because x^11 + x^9 + x^7 + x^5 + x^3 + x + 1 is irreducible over GF(2) (and 11=2*5+1).

PROG

(PARI) forstep(d=1, 10^5, 2, p=(1+sum(t=0, d, x^(2*t+1))); if(polisirreducible(Mod(1, 2)*p), print1(d, ", ")));

(Sage)

p = 1;

P.<x> = GF(2)[]

for n in range(1, 10^5, 2):

    p = p + x^(2*(n-1)+1) + x^(2*n+1);

    if p.is_irreducible():

        print(n)

# Joerg Arndt, Apr 28 2012

CROSSREFS

Sequence in context: A262602 A133847 A134180 * A249412 A098758 A275254

Adjacent sequences:  A107217 A107218 A107219 * A107221 A107222 A107223

KEYWORD

nonn,hard,more

AUTHOR

Joerg Arndt, Jun 08 2005

EXTENSIONS

More terms by Joerg Arndt, Apr 02 2011 and (terms >=2519), Apr 27 2012

STATUS

approved

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Last modified June 22 23:15 EDT 2017. Contains 288633 sequences.