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A087610 Number of (-1,0,1) polynomials of degree-n irreducible over the integers. 6
3, 5, 12, 34, 104, 292, 916, 2791, 8660, 26538, 81584, 248554 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A (-1,0,1) polynomial is defined as a monic polynomial whose remaining coefficients are either -1, 0, or 1. For each n, there are 3^n polynomials to consider.

LINKS

Table of n, a(n) for n=1..12.

Eric Weisstein's World of Mathematics, Irreducible Polynomial

EXAMPLE

a(2) = 5 because 1+x+x^2, 1+x^2, 1-x+x^2, -1+x+x^2, -1-x+x^2 are irreducible over the integers.

MAPLE

F:= proc(n) local T, count, t, x, p;

      if n::odd then

        T:= combinat:-cartprod([[-1, 0, 1]$(n-1), [1]])

      else

        T:= combinat:-cartprod([[-1, 0, 1]$(n-1), [-1, 1]])

      fi;

      count:= 0;

      while not T[finished] do

         t:= T[nextvalue]();

         p:= x^n + add(t[i]*x^(n-i), i=1..n);

         if irreduc(p) then count:= count+1 fi;

      od;

      if n::odd then 2*count else count fi;

end proc:

3, seq(F(n), n=2..11); # Robert Israel, Dec 10 2015

MATHEMATICA

Irreducible[p_, n_] := Module[{f}, f=FactorList[p, Modulus->n]; Length[f]==1 || Simplify[p-f[[2, 1]]]===0]; Table[xx=x^Range[0, n-1]; cnt=0; Do[p=x^n+xx.(IntegerDigits[i, 3, n]-1); If[Irreducible[p, 0], cnt++ ], {i, 0, 3^n-1}]; cnt, {n, 10}]

CROSSREFS

Cf. A087481 (irreducible polynomials of the form x^n +- x^(n-1) +- x^(n-2) +- ... +- 1), A087482 (irreducible binary polynomials).

Sequence in context: A002905 A220832 A323270 * A243013 A191636 A267337

Adjacent sequences:  A087607 A087608 A087609 * A087611 A087612 A087613

KEYWORD

nonn,more

AUTHOR

T. D. Noe, Sep 11 2003

EXTENSIONS

a(11) and a(12) from Robert Israel, Dec 10 2015

STATUS

approved

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Last modified June 24 14:20 EDT 2021. Contains 345417 sequences. (Running on oeis4.)