A smooth envelope of a topological algebra is introduced, and the following result is announced: the smooth envelope of a given subalgebra A in C ∞ (M ) coincides with C ∞ (M ) if and only if A has the same tangent bundle as M .In [15], J. L. Taylor introduced an operation now called the Arens-Michael envelope. It turns any topological algebra A into the projective limit A ♥ of its Banach quotient algebras (see [8], [14]). The numerous connections of this construction with complex analysis allow one to treat A ♥ as the holomorphic algebra 1 closest to A from the outside. This view is based, in particular, on the formula due to A. Yu. Pirkovskii [14] that connects the algebra P(M ) of polynomials on an affine algebraic variety M with the algebra O(M ) of holomorphic functions on M :(1)In the theory of involutive topological algebras, there is a similar construction which is called the pro-C * -envelope, which turns a topological algebra A into the projective limit Env C A of its C * -quotient algebras (see [7], [9], [13], [10]). The envelope Env C A can be treated as the continuous algebra 2 nearest to A on the outside. The analogue of (1) connects the algebra C(M ) of continuous functions on a paracompact locally compact space M with its arbitrary subalgebra A that has M as the involutive spectrum (see [1]):Both envelopes are used in generalizations of Pontryagin duality from commutative locally compact groups to the classes of noncommutative groups, in particular quantum groups ([3], [10]). Philosophically these theories are in some sense projections of functional analysis into the "noncommutative complex analysis" (considered as a geometric interpretation of the theory of Banach algebras, see [14]) and into the "noncommutative topology" (considered as a geometric interpretation of the theory of C * -algebras, see [6]).The operations ♥ and Env C and the corresponding duality theories give rise to the hypothesis that there are other similar operations, in particular, envelopes that generate by analogy a projection of functional analysis into the "noncommutative differential geometry". In this paper, we describe such an envelope. We define the C ∞ -envelope Env C ∞ A of an involutive topological algebra A and announce a formula analogous to (1) and (2):In this approach, holomorphic algebras are defined by the equality A ♥ = A. In the ideology of this branch of noncommutative geometry, it is assumed that this describes the properties of ordinary algebras O(M ) of holomorphic functions essential for generalizations. 2 Continuous algebras are defined by the equality EnvCA = A, and as in the case of holomorphic algebras, it is assumed that this distinguishes the properties of ordinary algebras C(M ) of continuous functions essential for generalizations.
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