A344198 Numbers m > 9 such that x^m + x^9 + 1 is irreducible over GF(2) while x^m + x^9 + x^6 + x^3 + 1 = x^m + (x^3 + 1)^3 is not.

12, 18, 30, 36, 54, 60, 84, 156, 198, 390, 540, 1950, 4788

Offset

1,1

Comments

Conjecture: Given e >= 0, odd numbers r, k > 0, a > 2^e*r*k, consider the following two statements:

(A) x^m + (x^k + 1)^(2^e*r) is irreducible over GF(2);

(B) x^m + x^(2^e*r*k) + 1 is irreducible over GF(2),

then:

(i) (A) implies (B);

(ii) if (B) is true and (A) is false, then:

(a) gcd(m,r) > 1;

(b) if prime p | gcd(m,r*k), then p*ord_p(2) | m;

(c) if e > 0, then m is odd.

Here ord(2,p) is the multiplicative order of 2 modulo p.

In other words, assuming that (B) is true, (A) is false if and only if (a), (b), (c) hold. (For the "if" part, note that if d = gcd(m,2^e*r) > 1 then x^m + (x^k + 1)^(2^e*r) must be reducible, since it is divisible by x^(m/d) + (x^k + 1)^(2^e*r/d).)

Here is the case r = 3, k = 3, e = 0, and (ii) means that m is in this sequence if and only if x^m + x^9 + 1 is irreducible and m is a multiple of 6.

If the conjecture is true, this is also numbers m >= 10 such that x^m + x^9 + 1 is irreducible over GF(2) while x^m + x^9 + x^8 + x + 1 is not (the case r = 9, k = 1, e = 0).

Links

Table of n, a(n) for n=1..13.

Example

18 is a term because x^18 + x^9 + 1 is irreducible over GF(2) but x^18 + x^9 + x^6 + x^3 + 1 is not: x^18 + x^9 + x^6 + x^3 + 1 = (x^6 + x^3 + 1)*(x^12 + x^9 + 1).

Prog

(PARI) isA344198(n) = polisirreducible(Mod(x^n+x^9+1, 2)) && !polisirreducible(Mod(x^n+x^9+x^6+x^3+1, 2))

Crossrefs

Similar sequences: A344177 (r=3, k=1), this sequence (r=3, k=3), A344199 (r=3, k=5), A344197 (r=5, k=1), A344200 (r=5, k=3).

Sequence in context: A124269 A361080 A179192 * A112054 A225576 A275082

Adjacent sequences: A344195 A344196 A344197 * A344199 A344200 A344201

Keyword

nonn,hard,more

Author

Jianing Song, May 11 2021

Status

approved