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).

1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 5, 7, 9, 9, 13, 11, 17, 19, 33

Offset: 1

Author

David A. Madore, Oct 28 2012

Keywords

• nonn

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.

Examples

			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.
		

Programs

Sage

R. = 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()