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A218452 Number of ways to factor (1 + x + x^2+ ... + x^(n - 1))^2 as the product of two monic polynomials of degree n - 1 with positive coefficients (counting order). 0
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 5, 7, 9, 9, 13, 11, 17, 19, 33 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,12

COMMENTS

a(n) is the number of ways one can divide the unit square in n possibly irregular lines * n possibly irregular columns (parallel to the sides) so that each of the diagonals of the n X n irregular checkerboard thus constructed has the same area as it would in a regular checkerboard. Alternatively, this is the number of ways to construct a pair of n-sided dice (probability distribution on the n sides, labeled 0 through n-1), no face having probability 0, so that the sum of the two dice follows the expected probability distribution for the sum of two fair n-sided dice. Note that a(n) is always odd because there is always the obvious factorization of (1+x+...+x^(n-1))^2 as 1+x+...+x^(n-1) times itself, and each other factorization counts twice.

LINKS

Table of n, a(n) for n=1..21.

EXAMPLE

For n=12 we have a(n)=3 because apart from the obvious factorization of (1+x+...+x^11)^2 as (1+x+...+x^11) times itself, there exist the factorizations p*q and q*p where p = (1-sqrt(3)*x+x^2) * (1-x+x^2) * (1+x^2) * (1+x+x^2)^2 * (1+x) and q = (1-sqrt(3)*x+x^2) * (1-x+x^2) * (1+x^2) * (1+sqrt(3)*x+x^2)^2 * (1+x), both of which have positive coefficients, and those are the only two possible.

PROG

(Sage)

R.<x> = AA['x']

def has_positive_coefficients(pol):

    return not any(c <= 0 for c in pol.coeffs())

def trydie(m):

    results = []

    tmp = list(factor(sum([x^i for i in range(m)])))

    facs = [f for (f, _) in tmp]

    n = len(facs)

    for i in range((3^n+1)//2):

        exps = [(i//(3^k))%3 for k in range(n)]

        coexps = [2-v for v in exps]

        pol = R(prod([facs[k]^exps[k] for k in range(n)]))

        copol = R(prod([facs[k]^coexps[k] for k in range(n)]))

        if pol.degree()<m and copol.degree()<m and has_positive_coefficients(pol) and has_positive_coefficients(copol):

            pol /= pol.subs({x:1})

            copol /= copol.subs({x:1})

            results += [(pol.coeffs(), copol.coeffs())]

    return 2*len(results)-1

CROSSREFS

Sequence in context: A265527 A217250 A213923 * A007731 A306590 A249494

Adjacent sequences:  A218449 A218450 A218451 * A218453 A218454 A218455

KEYWORD

nonn

AUTHOR

David A. Madore, Oct 28 2012

STATUS

approved

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Last modified October 23 08:40 EDT 2021. Contains 348211 sequences. (Running on oeis4.)