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Homogenization of a polynomial : ウィキペディア英語版
Homogeneous polynomial
In mathematics, a homogeneous polynomial is a polynomial whose nonzero terms all have the same degree.〔D. Cox, J. Little, D. O'Shea: ''Using Algebraic Geometry'', 2nd ed., page 2. Springer-Verlag, 2005.〕 For example, x^5 + 2 x^3 y^2 + 9 x y^4 is a homogeneous polynomial of degree 5, in two variables; the sum of the exponents in each term is always 5. The polynomial x^3 + 3 x^2 y + z^7 is not homogeneous, because the sum of exponents does not match from term to term. A polynomial is homogeneous if and only if it defines a homogeneous function. An algebraic form, or simply form, is a function defined by a homogeneous polynomial.〔However, as some authors do not make a clear distinction between a polynomial and its associated function, the terms ''homogeneous polynomial'' and ''form'' are sometimes considered as synonymous.〕 A binary form is a form in two variables. A ''form'' is also a function defined on a vector space, which may be expressed as a homogeneous function of the coordinates over any basis.
A polynomial of degree 0 is always homogeneous; it is simply an element of the field or ring of the coefficients, usually called a constant or a scalar. A form of degree 1 is a linear form.〔''Linear forms'' are defined only for finite-dimensional vector space, and have thus to be distinguished from ''linear functionals'', which are defined for every vector space. "Linear functional" is rarely used for finite-dimensional vector spaces.〕 A form of degree 2 is a quadratic form. In geometry, the Euclidean distance is the square root of a quadratic form.
Homogeneous polynomials are ubiquitous in mathematics and physics.〔Homogeneous polynomials in physics often appear as a consequence of dimensional analysis, where measured quantities must match in real-world problems.〕 They play a fundamental role in algebraic geometry, as a projective algebraic variety is defined as the set of the common zeros of a set of homogeneous polynomials.
==Properties==
A homogeneous polynomial defines a homogeneous function. This means that a multivariate polynomial ''P'' is homogeneous of degree ''d'' if and only if
:P(\lambda x_1, \ldots, \lambda x_n)=\lambda^d\,P(x_1,\ldots,x_n)\,,
for every \lambda in the field of the coefficients. In particular, if ''P'' is homogeneous then
:P(x_1,\ldots,x_n)=0 \quad\Rightarrow\quad P(\lambda x_1, \ldots, \lambda x_n)=0,
for every \lambda. This property is fundamental in the definition of a projective variety.
Any nonzero polynomial may be decomposed, in a unique way, as a sum of homogeneous polynomials of different degrees, which are called the homogeneous components of the polynomial.
Given a polynomial ring R=K(\ldots,x_n ) over a field (or, more generally, a ring) ''K'', the homogeneous polynomials of degree ''d'' form
a vector space (or a module), commonly denoted R_d. The above unique decomposition means that R is the direct sum of the R_d (sum over all nonnegative integers).
The dimension of the vector space (or free module) R_d is the number of different monomials of degree ''d'' in ''n'' variables (that is the maximal number of nonzero terms in a homogeneous polynomial of degree ''d'' in ''n'' variables). It is equal to the binomial coefficient
:\binom=\binom=\frac.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「Homogeneous polynomial」の詳細全文を読む



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