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Quasisymmetric function : ウィキペディア英語版
Quasisymmetric function

In algebra and in particular in algebraic combinatorics, a quasisymmetric function is any element in the ring of quasisymmetric functions which is in turn a subring of the formal power series ring with a countable number of variables. This ring generalizes the ring of symmetric functions. This ring can be realized as a specific limit of the rings of quasisymmetric polynomials in ''n'' variables, as ''n'' goes to infinity. This ring serves as universal structure in which relations between quasisymmetric polynomials can be expressed in a way independent of the number ''n'' of variables (but its elements are neither polynomials nor functions).
== Definitions ==

The ring of quasisymmetric functions, denoted QSym, can be defined over any commutative ring ''R'' such as the integers.
Quasisymmetric
functions are power series of bounded degree in variables x_1,x_2,x_3, \dots with coefficients in ''R'', which are shift invariant in the sense that the coefficient of the monomial x_1^x_2^ \cdots x_k^ is equal to the coefficient of the monomial x_^ x_^\cdots x_^ for any strictly increasing sequence of positive integers
i_1< i_2< \cdots < i_k indexing the variables and any positive integer sequence (\alpha_1, \alpha_2,\ldots,\alpha_k) of exponents.〔
Stanley, Richard P. ''Enumerative Combinatorics'', Vol. 2, Cambridge University Press, 1999. ISBN 0-521-56069-1 (hardback) ISBN 0-521-78987-7 (paperback).〕
Much of the study of quasisymmetric functions is based on that of symmetric functions.
A quasisymmetric function in finitely many variables is a ''quasisymmetric polynomial''.
Both symmetric and quasisymmetric polynomials may be characterized in terms of actions of the symmetric group S_n^,\dots, x_n.
One such action of S_n permutes variables,
changing a polynomial p(x_1^)
of variables having consecutive indices.
Those polynomials unchanged by all such swaps
form the subring of symmetric polynomials.
A second action of S_n conditionally permutes variables,
changing a polynomial p(x_1,\ldots,x_n)
by swapping pairs (x_i^
x_1^2 x_2 x_3 + x_1^2 x_2 x_4 + x_1^2 x_3 x_4 + x_2^2 x_3 x_4
+ x_1 x_2^2 x_3 + x_1 x_2^2 x_4 + x_1 x_3^2 x_4 + x_2 x_3^2 x_4 \\
{} + x_1 x_2 x_3^2 + x_1 x_2 x_4^2 + x_1 x_3 x_4^2 + x_2 x_3 x_4^2. \,
\end{align}


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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