Talk:Fréchet algebra

Expert needed
I added Springer EoM as a reference but the definition used there does not match the definition used in the article. The definition used in the article seems to match the definition of m-convex B0-algebra which Springer says is the definition of Fréchet algebra used by "some authors". An expert is needed to add the Springer definition or at least add a redirect to an article that has it.--RDBury (talk) 20:57, 11 February 2010 (UTC)

Michael's problem
Is it still an open problem? What about the proof by Berit Stensones? http://www.math.lsa.umich.edu/~berit/michaelnov98.ps —Preceding unsigned comment added by Jaan Vajakas (talk • contribs) 23:47, 15 May 2010 (UTC)

A google search shows that the article has not been published but the preprint has been referenced in some works. E. g. http://eom.springer.de/b/b120050.htm tells that "the Michael problem has an affirmative solution" and cites Stensones' paper. http://wwwmath.uni-muenster.de/u/walther.paravicini/files/essay.pdf discusses the problem and says about Stensones' work that "there is hope that the proof is correct". --Jaan Vajakas (talk) 00:20, 16 May 2010 (UTC)

The paper is over 10 years old by now. Hard to believe it would take that long to referee even if it is very technical. So there must be some problems with it. I would regard the conjecture therefore as still open. — Preceding unsigned comment added by 130.75.46.166 (talk) 13:10, 26 October 2011 (UTC)

Today I was told by two professors in the field (W. Želazko and M. Abel) that indeed a gap was found in her proof and the problem is still open. --Jaan Vajakas (talk) 13:33, 30 May 2012 (UTC)

=Rough Draft Fréchet Algebra=

In mathematics, especially functional analysis, a Fréchet algebra, named after Maurice René Fréchet, is an associative algebra $$A$$ over the real or complex numbers that at the same time is also a (locally convex) Fréchet space. The multiplication operation $$ (a,b) \mapsto a*b$$ for $$ a,b \in A $$ is required to be jointly continuous. If $$\{ \| \cdot \|_n \}_{n=0}^\infty$$ is an increasing family of seminorms for the topology of $$A$$, the joint continuity of multiplication is equivalent to there being a constant $$C_n >0$$ and integer $$m \ge n$$ for each $$n$$ such that $$\| a b \|_n \leq C_n \| a \|_m \|b \|_m$$ for all $$ a, b \in A$$. Fréchet algebras are also called B0-algebras.

A Fréchet algebra is $$m$$-convex if there exists such a family of semi-norms for which $$m=n$$. In that case, by rescaling the seminorms, we may also take $$C_n=1$$ for each $$n$$ and the seminorms are said to be submultiplicative: $$ \| a b \|_n \leq \| a \|_n \| b \|_n$$ for all $$a, b \in A$$. $$m$$-convex Fréchet algebras may also be called Fréchet algebras.

A Fréchet algebra may or may not have an identity element $$ 1_A $$. If $$ A $$ is unital, we do not require that $$ \|1_A\|_n=1 $$, as is often done for Banach algebras.

Lschweitz (talk) 22:25, 4 May 2011 (UTC)

Properties
  Continuity of multiplication. Multiplication is separately continuous if $$a_k \rightarrow a$$ implies $$a_k b \rightarrow ab$$ and $$ba_k \rightarrow ba$$ for every $$a, b \in A$$ and sequence $$a_k \rightarrow a$$ converging in the Fr&eacute;chet topology of $$A$$. Multiplication is jointly continuous if $$a_k \rightarrow a$$ and $$b_k \rightarrow b$$ imply $$a_k b_k \rightarrow ab$$. Joint continuity of multiplication follows from the definition of a Fr&eacute;chet algebra. For a Fr&eacute;chet space with an algebra structure, if the multiplication is separately continuous, then it is automatically jointly continuous, Chapter VII, Proposition 1, , $$\S$$ 2.9.   Group of invertible elements. If $$invA$$ is the set of invertible elements of $$A$$, then the inverse map $$u \mapsto u^{-1}$$, $$invA \rightarrow invA$$ is continuous if and only if $$invA$$ is a $G_\delta$ set, Chapter VII, Proposition 2. Unlike for Banach algebras, $$inv A$$ may not be an open set. If $$inv A$$ is open, then $$A$$ is called a $$Q$$-algebra. (If $$A$$ happens to be non-unital, then we may adjoin a unit to $$A$$ and work with $$inv A^+$$, or the set of quasi invertibles may take the place of $$inv A$$.)    Conditions for $$m$$-convexity. A Fréchet algebra is $$m$$-convex if and only if for every increasing family $$\{ \| \cdot \|_n \}_{n=0}^\infty$$ of seminorms which topologize $$A$$, for each $$m \in \mathbb N$$ there exists $$ p \geq m $$ and $$C_m>0$$  such that
 * $$ \| a_1 a_2 \cdots a_n \|_m \leq C_m^n \| a_1 \|_p \| a_2 \|_p \cdots \| a_n \|_p,$$

for all $$a_1, a_2, \dots a_n \in A$$ and $$n \in \mathbb N$$, Lemma 1.2. A commutative Fréchet $$Q$$-algebra is $$m$$-convex, Theorem 13.17. But there exist examples of non-commutative Fréchet $$Q$$-algebras which are not $$m$$-convex.   Properties of $$m$$-convex Fr&eacute;chet algebras.  A Fr&eacute;chet algebra is $$m$$-convex if and only if it is a countable projective limit of Banach algebras, Theorem 5.1. An element of $$A$$ is invertible if and only if it's image in each Banach algebra of the projective limit is invertible, Theorem 5.2.  

Lschweitz (talk) 05:44, 1 December 2015 (UTC)

Examples
  ''' Zero multiplication. ''' If $$E$$ is any Fréchet space, we can make a Fréchet algebra structure by setting $$e * f = 0$$ for all $$e, f \in E$$.   Smooth functions on the circle. Let $$\mathbb T$$ be the circle group, or 1-sphere. This is a 1-dimensional compact differentiable manifold, with no boundary. Let $$A=C^{\infty}(\mathbb T)$$ be the set of infinitely differentiable complex valued functions on $$\mathbb T$$. This is clearly an algebra over the complex numbers, for pointwise multiplication. (Use the product rule for differentiation.) It is commutative, and the constant function $$1$$ acts as an identity. Define a countable set of seminorms on $$A$$ by

\| \varphi \|_{n} = \| \varphi^{(n)} \|_{\infty}, \qquad \varphi \in A, $$ where $$\| \varphi^{(n)} \|_{\infty} = \sup_{x \in {\mathbb T}} |\varphi^{(n)}(x)|$$ denotes the supremum of the absolute value of the $$n$$th derivative $$\varphi^{(n)}$$. Then, by the product rule for differentiation, we have

\begin{align} \| \varphi \psi \|_{n} &=& \biggl\| \sum_{i = 0}^{n} {n \choose i} \varphi^{(i)} \psi^{(n-i)} \biggr\|_{\infty} & \leq & \sum_{i =0}^{n} {n \choose i} \| \varphi  \|_{i} \| \psi \|_{n-i} \\ & \leq & \sum_{i =0}^{n} {n \choose i} \| \varphi  \|_{n}^{\prime} \| \psi \|_{n}^{\prime} & = & 2^n \| \varphi \|_{n}^{\prime} \| \psi \|_{n}^{\prime}, \end{align} $$ where $${n \choose i}$$ denotes the binomial coefficient $${n! \over{i! (n-i)!}}$$, and $$\| \cdot \|_{n}^{\prime} = \max_{k \leq n} \| \cdot \|_{k}$$. The primed seminorms are submultiplicative after re-scaling by $$C_n=2^n$$.    Sequences on $$\N$$. Let $$\Complex^\N$$ be the space of complex-valued sequences  on the natural numbers $$\N$$. Define increasing seminorms by $$\| \varphi \|_n = \max_{k\leq n} |\varphi(k)|$$. With pointwise multiplication, $$\Complex^\N$$ is a commutative Fréchet algebra. In fact, each seminorm is submultiplicative $$ \| \varphi \psi \|_n \leq \| \varphi \|_n \| \psi \|_n$$ for all $$ \varphi, \psi \in A $$. This $$m$$-convex Fréchet algebra is unital, since the constant sequence $$1(k) = 1$$, $$k \in \N$$ is in $$A$$. </li> <li> Equipped with the topology of uniform convergence on compact sets, and pointwise multiplication, $$C(\mathbb C)$$, the algebra of all continuous functions on the complex plane $$\mathbb C$$, or to the algebra $$Hol(\mathbb C)$$ of holomorphic functions on $$\mathbb C$$. </li> <li>  Convolution algebra of rapidly vanishing functions on a finitely generated discrete group. Let $$G$$ be a finitely generated group, with the discrete topology. This means that there exists a set of finitely many elements $$U= \{ g_{1}, \dots g_{n}\} \subseteq G$$ such that the union of all products $$\bigcup_{n=0}^{\infty} U^n$$ equals $$G$$. Without loss of generality, we may also assume that the identity element $$e$$ of $$G$$ is contained in $$U$$. Define a function $$\ell \colon G \rightarrow [0, \infty)$$ by

\ell(g) = \min \{ n \mid g \in U^n \}. $$ Then $$\ell(gh ) \leq \ell(g) + \ell(h)$$, and $$\ell(e) = 0$$, since we define $$U^{0} = \{ e \}$$. Let $$A$$ be the $$\mathbb C$$-vector space

S(G) = \biggr\{ \varphi\colon G \rightarrow {\mathbb C} \,\,\biggl|\,\, \| \varphi \|_{d} < \infty, \quad d = 0,1, 2, \dots \biggr\}, $$ where the seminorms $$\| \cdot \|_{d}$$ are defined by

\| \varphi \|_{d} = \| \ell^d \varphi \|_{1} = \sum_{g \in G} \ell(g)^d |\varphi(g)|. $$

$$A$$ is an $$m$$-convex Fr&eacute;chet algebra for the convolution multiplication

\varphi {*} \psi (g) = \sum_{h \in G} \varphi(h) \psi(h^{-1}g), $$ $$A$$ is unital because $$G$$ is discrete, and $$A$$ is commutative if and only if $$G$$ is Abelian. </li> <li>  Non $$m$$-convex Fréchet algebras. The Aren's algebra $$A=L^\omega[0,1]= \bigcup_{p \geq 1} L^p[0,1]$$ is an example of a commutative non-$$m$$-convex Fréchet  algebra with discontinuous inversion. The topology is given by $L^p$ norms

\| f \|_p = \biggl( \int_0^1 | f(t) |^p dt \biggr)^{1/p}, \qquad f \in A, $$ and multiplication is given by convolution of functions with respect to Lebesgue measure on $$[0,1]$$, Example 6.13 (2). </li> </ol> Lschweitz (talk) 22:23, 4 May 2011 (UTC)

Generalizations
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  or an F-space.

If the requirement that the number of seminorms be countable is dropped, the algebra becomes locally convex (LC) or locally multiplicatively convex (LMC). A complete LMC algebra is called an Arens-Michael algebra.

Lschweitz (talk) 01:08, 23 November 2015 (UTC)

Open problems
Perhaps themost famous, still open problem of the theory of topological algebras is whether all linear multiplicative functionals on a Frechet algebra are continuous. The statement that this be the case is known as Michael's Conjecture.

Lschweitz (talk) 05:36, 30 November 2015 (UTC)