A Fréchet algebra is -convex if there exists such a family of semi-norms for which . In that case, by rescaling the seminorms, we may also take for each and the
seminorms are said to be submultiplicative: for all .[3]-convex Fréchet algebras may also be called Fréchet algebras (Husain 1991) (Żelazko 2001) harv error: no target: CITEREFŻelazko2001 (help).
Continuity of multiplication. Multiplication is separately continuous if and for every and sequence converging in the Fréchet topology of . Multiplication is jointly continuous if and imply . Joint continuity of multiplication is part of the definition of a Fréchet algebra. For a Fréchet space with an algebra structure, if the multiplication is separately continuous, then it is automatically jointly continuous (Waelbroeck 1971, Chapter VII, Proposition 1), (Palmer 1994, 2.9).
Group of invertible elements. If is the set of invertible elements of , then the inverse map , is continuous if and only if is a set (Waelbroeck 1971, Chapter VII, Proposition 2). Unlike for Banach algebras, may not be an open set. If is open, then is called a -algebra. (If happens to be non-unital, then we may adjoin a unit to [4] and work with , or the set of quasi invertibles[5] may take the place of .)
Conditions for -convexity. A Fréchet algebra is -convex if and only if for every, if and only if for one, increasing family of seminorms which topologize , for each there exists and such that
for all and (Mitiagin et al. 1962, Lemma 1.2) harv error: no target: CITEREFMitiagin_et_al.1962 (help). A commutative Fréchet -algebra is -convex (Żelazko 1965, Theorem 13.17). But there exist examples of non-commutative Fréchet -algebras which are not -convex (Żelazko 1994).
Properties of -convex Fréchet algebras. A Fréchet algebra is -convex if and only if it is a countableprojective limit of Banach algebras (Michael 1952, Theorem 5.1). An element of is invertible if and only if it's image in each Banach algebra of the projective limit is invertible (Michael 1952, Theorem 5.2).[6] See also (Palmer 1994, Theorem 2.9.6).
Examples
Zero multiplication. If is any Fréchet space, we can make a Fréchet algebra structure by setting for all .
where denotes the supremum of the absolute value of the th derivative .[7] Then, by the product rule for differentiation, we have
where denotes the binomial coefficient, and . The primed seminorms are submultiplicative after re-scaling by .
Sequences on . Let be the space of complex-valued sequences on the natural numbers. Define an increasing family of seminorms on by . With pointwise multiplication, is a commutative Fréchet algebra. In fact, each seminorm is submultiplicative for . This -convex Fréchet algebra is unital, since the constant sequence , is in .
Convolution algebra of rapidly vanishing functions on a finitely generated discrete group. Let be a finitely generated group, with the discrete topology. This means that there exists a set of finitely many elements such that the union of all products equals . Without loss of generality, we may also assume that the identity element of is contained in . Define a function by
is an -convex Fréchet algebra for the convolution
multiplication
[10] is unital because is discrete, and is commutative if and only if is Abelian.
Non -convex Fréchet algebras. The Aren's algebra is an example of a commutative non--convex Fréchet algebra with discontinuous inversion. The topology is given by norms
We can drop the requirement for the algebra to be locally convex, but still a complete metric space. In this case, the underlying space may be called a Fréchet space (Waelbroeck 1971) or an F-space (Rudin 1973, 1.8(e)).
If the requirement that the number of seminorms be countable is dropped, the algebra becomes locally convex (LC) or locally multiplicatively convex (LMC) (Michael 1952) (Husain 1991). A complete LMC algebra is called an Arens-Michael algebra (Fragoulopoulou 2005, Chapter 1).
Open problems
Perhaps the most famous, still open problem of the theory of topological algebras is whether all linear multiplicative functionals on an -convex Frechet algebra are continuous. The statement that this be the case is known as Michael's Conjecture (Michael 1952, 12, Question 1) (Palmer 1994, 3.1).
^Joint continuity of multiplication means that for every absolutely convexneighborhood of zero, there is an absolutely convex neighborhood of zero for which , from which the seminorm inequality follows. Conversely,
^In other words, an -convex Fréchet algebra is a topological algebra, in which the topology is given by a countable family of submultiplicative seminorms: p(fg) ≤ p(f)p(g), and the algebra is complete.
^If is an algebra over a field , the unitization of is the direct sum , with multiplication defined as
^If is non-unital, replace invertible with quasi-invertible.
^To see the completeness, let be a Cauchy sequence. Then each derivative is a Cauchy sequence in the sup norm on , and hence converges uniformly to a continuous function on . It suffices to check that is the th derivative of . But, using the fundamental theorem of calculus, and taking the limit inside the integral (using uniform convergence), we have
^We can replace the generating set with , so that . Then satisfies the additional property , and is a length function on .
^To see that is Fréchet space, let be a Cauchy sequence. Then for each , is a Cauchy sequence in . Define to be the limit. Then
where the sum ranges over any finite subset of .
Let , and let be such that
for . By letting run, we have
for . Summing over all of , we therefore
have for . By the estimate
we obtain . Since this holds for each , we have and
in the Fréchet topology, so is complete.
Husain, Taqdir (1991), Orthogonal Schauder Bases, Pure and Applied Mathematics, vol. 143, New York: Marcel Dekker, Inc., ISBN0-8247-8508-8.
Michael, Ernest A. (1952), Locally Multiplicatively-Convex Topological Algebras, Memoirs of the American Mathematical Society, vol. 11, MR0051444.
Mitiagin, B.; Rolewicz, S.; Żelazko, W. (1962), "Entire functions in B0-algebras", Studia Mathematica, 21: 291–306, MR0144222.
Palmer, T.W. (1994), Banach Algebras and the General Theory of *-algebras, Volume I: Algebras and Banach Algebras, Encyclopedia of Mathematics and its Applications, vol. 49, New York: Cambridge University Press, ISBN978-0-521-36637-3.
Rudin, Walter (1973), Functional Analysis, Series in Higher Mathematics, New York: McGraw-Hill Book Company, ISBN978-0-070-54236-5.