In algebra, the ring of restricted power series is the subring of a formal power series ring that consists of power series whose coefficients approach zero as degree goes to infinity.^{[1]} Over a non-archimedean complete field, the ring is also called a Tate algebra. Quotient rings of the ring are used in the study of a formal algebraic space as well as rigid analysis, the latter over non-archimedean complete fields.
Over a discrete topological ring, the ring of restricted power series coincides with a polynomial ring; thus, in this sense, the notion of "restricted power series" is a generalization of a polynomial.
Let A be a linearly topologized ring, separated and complete and the fundamental system of open ideals. Then the ring of restricted power series is defined as the projective limit of the polynomial rings over :
In other words, it is the completion of the polynomial ring with respect to the filtration . Sometimes this ring of restricted power series is also denoted by .
Clearly, the ring can be identified with the subring of the formal power series ring that consists of series with coefficients ; i.e., each contains all but finitely many coefficients . Also, the ring satisfies (and in fact is characterized by) the universal property:^{[4]} for (1) each continuous ring homomorphism to a linearly topologized ring , separated and complete and (2) each elements in , there exists a unique continuous ring homomorphism
extending .
In rigid analysis, when the base ring A is the valuation ring of a complete non-archimedean field , the ring of restricted power series tensored with ,
is called a Tate algebra, named for John Tate.^{[5]} It is equivalently the subring of formal power series which consists of series convergent on , where is the valuation ring in the algebraic closure .
The maximal spectrum of is then a rigid-analytic space that models an affine space in rigid geometry.
Define the Gauss norm of in by
This makes a Banach algebra over k; i.e., a normed algebra that is complete as a metric space. With this norm, any ideal of is closed^{[6]} and thus, if I is radical, the quotient is also a (reduced) Banach algebra called an affinoid algebra.
Some key results are:
As consequence of the division, preparation theorems and Noether normalization, is a Noetherian unique factorization domain of Krull dimension n.^{[11]} An analog of Hilbert's Nullstellensatz is valid: the radical of an ideal is the intersection of all maximal ideals containing the ideal (we say the ring is Jacobson).^{[12]}
Results for polynomial rings such as Hensel's lemma, division algorithms (or the theory of Gröbner bases) are also true for the ring of restricted power series. Throughout the section, let A denote a linearly topologized ring, separated and complete.