111 research outputs found
Counting decomposable univariate polynomials
A univariate polynomial f over a field is decomposable if it is the
composition f = g(h) of two polynomials g and h whose degree is at least 2. We
determine the dimension (over an algebraically closed field) of the set of
decomposables, and an approximation to their number over a finite field. The
tame case, where the field characteristic p does not divide the degree n of f,
is reasonably well understood, and we obtain exponentially decreasing error
bounds. The wild case, where p divides n, is more challenging and our error
bounds are weaker
Survey on counting special types of polynomials
Most integers are composite and most univariate polynomials over a finite
field are reducible. The Prime Number Theorem and a classical result of
Gau{\ss} count the remaining ones, approximately and exactly.
For polynomials in two or more variables, the situation changes dramatically.
Most multivariate polynomials are irreducible. This survey presents counting
results for some special classes of multivariate polynomials over a finite
field, namely the the reducible ones, the s-powerful ones (divisible by the
s-th power of a nonconstant polynomial), the relatively irreducible ones
(irreducible but reducible over an extension field), the decomposable ones, and
also for reducible space curves. These come as exact formulas and as
approximations with relative errors that essentially decrease exponentially in
the input size.
Furthermore, a univariate polynomial f is decomposable if f = g o h for some
nonlinear polynomials g and h. It is intuitively clear that the decomposable
polynomials form a small minority among all polynomials. The tame case, where
the characteristic p of Fq does not divide n = deg f, is fairly
well-understood, and we obtain closely matching upper and lower bounds on the
number of decomposable polynomials. In the wild case, where p does divide n,
the bounds are less satisfactory, in particular when p is the smallest prime
divisor of n and divides n exactly twice. The crux of the matter is to count
the number of collisions, where essentially different (g, h) yield the same f.
We present a classification of all collisions at degree n = p^2 which yields an
exact count of those decomposable polynomials.Comment: to appear in Jaime Gutierrez, Josef Schicho & Martin Weimann
(editors), Computer Algebra and Polynomials, Lecture Notes in Computer
Scienc
Permanent and determinant
AbstractThe n×n permanent is not a projection of the m×m determinant if m ⩽ √2n− 6√n
Counting reducible and singular bivariate polynomials
AbstractAmong the bivariate polynomials over a finite field, most are irreducible. We count some classes of special polynomials, namely the reducible ones, those with a square factor, the “relatively irreducible” ones which are irreducible but factor over an extension field, and the singular ones, which have a root at which both partial derivatives vanish
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