In mathematics, a Laurent polynomial (named after Pierre Alphonse Laurent) in one variable over a field is a linear combination of positive and negative powers of the variable with coefficients in . Laurent polynomials in X form a ring denoted [X, X^{−1}].^{[1]} They differ from ordinary polynomials in that they may have terms of negative degree. The construction of Laurent polynomials may be iterated, leading to the ring of Laurent polynomials in several variables.
Definition
A Laurent polynomial with coefficients in a field is an expression of the form
where X is a formal variable, the summation index k is an integer (not necessarily positive) and only finitely many coefficients p_{k} are nonzero. Two Laurent polynomials are equal if their coefficients are equal. Such expressions can be added, multiplied, and brought back to the same form by reducing similar terms. Formulas for addition and multiplication are exactly the same as for the ordinary polynomials, with the only difference that both positive and negative powers of X can be present:
and
Since only finitely many coefficients a_{i} and b_{j} are nonzero, all sums in effect have only finitely many terms, and hence represent Laurent polynomials.
Properties
 A Laurent polynomial over C may be viewed as a Laurent series in which only finitely many coefficients are nonzero.
 The ring of Laurent polynomials R[X, X^{−1}] is an extension of the polynomial ring R[X] obtained by "inverting X". More rigorously, it is the localization of the polynomial ring in the multiplicative set consisting of the nonnegative powers of X. Many properties of the Laurent polynomial ring follow from the general properties of localization.
 If R is an integral domain, the units of the Laurent polynomial ring R[X, X^{−1}] have the form uX^{k}, where u is a unit of R and k is an integer. In particular, if K is a field then the units of K[X, X^{−1}] have the form aX^{k}, where a is a nonzero element of K.
 The Laurent polynomial ring R[X, X^{−1}] is isomorphic to the group ring of the group Z of integers over R. More generally, the Laurent polynomial ring in n variables is isomorphic to the group ring of the free abelian group of rank n. It follows that the Laurent polynomial ring can be endowed with a structure of a commutative, cocommutative Hopf algebra.
References
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