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Hung Le Pham

Publications and source records attributed to Hung Le Pham.

12 recordsLinked to original sources
arXiv2024

Constructing non-AMNM weighted convolution algebras for every semilattice of infinite breadth

The AMNM property for commutative Banach algebras is a form of Ulam stability for multiplicative linear functionals. We show that on any semilattice of infinite breadth, one may construct a weight for which the resulting weighted convolution algebra fails to have the AMNM property. Our work is the culmination of a trilogy started in [Semigroup Forum 102 (2021), no. 1, 86-103] and continued in [European J. Combin. 94 (2021), article 103311]. In particular, we obtain a refinement of the main result of the second paper, by establishing a dichotomy for union-closed set systems that has a Ramsey-theoretic flavour.

math.FA↗
arXiv2021

Unavoidable subprojections in union-closed set systems of infinite breadth

We consider union-closed set systems with infinite breadth, focusing on three particular configurations ${\mathcal T}_{\rm max}(E)$, ${\mathcal T}_{\rm min}(E)$ and ${\mathcal T}_{\rm ort}(E)$. We show that these three configurations are not isolated examples; in any given union-closed set system of infinite breadth, at least one of these three configurations will occur as a subprojection. This characterizes those union-closed set systems which have infinite breadth, and is the first general structural result for such set systems.

math.CO↗
arXiv2020

Stability of characters and filters for weighted semilattices

We continue the study of the AMNM property for weighted semilattices that was initiated in [Y. Choi, J. Austral. Math. Soc. 95 (2013), no. 1, 36-67; arXiv 1203.6691]. We reformulate this in terms of stability of filters with respect to a given weight function, and then provide a combinatorial condition which is necessary and sufficient for this "filter stability" property to hold. Examples are given to show that this new condition allows for easier and unified proofs of some results in [Choi, ibid.], and furthermore allows us to verify the AMNM property in situations not covered by the results of that paper. As a final application, we show that for a large class of semilattices, arising naturally as union-closed set systems, one can always construct weights for which the AMNM property fails.

math.FA↗
arXiv2015

Characterisations of Fourier and Fourier--Stieltjes algebras on locally compact groups

Motivated by the beautiful work of M. A. Rieffel (1965) and of M. E. Walter (1974), we obtain characterisations of the Fourier algebra $A(G)$ of a locally compact group $G$ in terms of the class of $F$-algebras (i.e. a Banach algebra $A$ such that its dual $A'$ is a $W^*$-algebra whose identity is multiplicative on $A$). For example, we show that the Fourier algebras are precisely those commutative semisimple $F$-algebras that are Tauberian, contain a nonzero real element, and possess a dual semigroup that acts transitively on their spectrums. Our characterisations fall into three flavours, where the first one will be the basis of the other two. The first flavour also implies a simple characterisation of when the predual of a Hopf-von Neumann algebra is the Fourier algebra of a locally compact group. We also obtain similar characterisations of the Fourier--Stieltjes algebras of $G$. En route, we prove some new results on the problem of when a subalgebra of $A(G)$ is the whole algebra and on representations of discrete groups.

math.FA↗
arXiv2015

Idempotents of small norm

Let $Γ$ be a locally compact group. We answer two questions left open in [7] and [9]: i) For abelian $Γ$, we prove that if $χ_S \in B(Γ)$ is an idempotent with norm $\left\|χ_S \right\| < \frac{4}{3}$, then $S$ is the union of two cosets of an open subgroup of $Γ$. ii) For general $Γ$, we prove that if $χ_S \in M_{cb}A(Γ)$ is an idempotent with norm $\left\| χ_S \right\|_{cb} < \frac{1 + \sqrt{2}}{2}$, then $S$ is an open coset in $Γ$.

math.FA↗
arXiv2013

Equivalence of multi-norms

The theory of multi-norms was developed by H.\ G.\ Dales and M.\ E.\ Polyakov in a memoir that was published in \emph{Dissertationes Mathematicae}. In that memoir, the notion of `equivalence' of multi-norms was defined. In the present memoir, we make a systematic study of when various pairs of multi-norms are mutually equivalent.

math.FA↗
arXiv2012

Multi-norms and the injectivity of $L^p(G)$

Let $G$ be a locally compact group, and take $p\in(1,\infty)$. We prove that the Banach left $L^1(G)$-module $L^p(G)$ is injective (if and) only if the group $G$ is amenable. Our proof uses the notion of multi-norms. We also develop the theory of multi-normed spaces.

math.FA↗
arXiv2012

Isometries between quantum convolution algebras

Given locally compact quantum groups $\G_1$ and $\G_2$, we show that if the convolution algebras $L^1(\G_1)$ and $L^1(\G_2)$ are isometrically isomorphic as algebras, then $\G_1$ is isomorphic either to $\G_2$ or the commutant $\G_2'$. Furthermore, given an isometric algebra isomorphism $θ:L^1(\G_2) \rightarrow L^1(\G_1)$, the adjoint is a *-isomorphism between $L^\infty(\G_1)$ and either $L^\infty(\G_2)$ or its commutant, composed with a twist given by a member of the intrinsic group of $L^\infty(\G_2)$. This extends known results for Kac algebras (although our proofs are somewhat different) which in turn generalised classical results of Wendel and Walter. We show that the same result holds for isometric algebra homomorphisms between quantum measure algebras (either reduced or universal). We make some remarks about the intrinsic groups of the enveloping von Neumann algebras of C$^*$-algebraic quantum groups.

math.OA↗
arXiv2008

Preduals of semigroup algebras

For a locally compact group $G$, the measure convolution algebra $M(G)$ carries a natural coproduct. In previous work, we showed that the canonical predual $C_0(G)$ of $M(G)$ is the unique predual which makes both the product and the coproduct on $M(G)$ weak$^*$-continuous. Given a discrete semigroup $S$, the convolution algebra $\ell^1(S)$ also carries a coproduct. In this paper we examine preduals for $\ell^1(S)$ making both the product and the coproduct weak$^*$-continuous. Under certain conditions on $S$, we show that $\ell^1(S)$ has a unique such predual. Such $S$ include the free semigroup on finitely many generators. In general, however, this need not be the case even for quite simple semigroups and we construct uncountably many such preduals on $\ell^1(S)$ when $S$ is either $\mathbb Z_+\times\mathbb Z$ or $(\mathbb N,\cdot)$.

math.FA↗
arXiv2008

Conditions implying the uniqueness of the weak$^*$-topology on certain group algebras

We investigate possible preduals of the measure algebra $M(G)$ of a locally compact group and the Fourier algebra $A(G)$ of a separable compact group. Both of these algebras are canonically dual spaces and the canonical preduals make the multiplication separately weak$^*$-continuous so that these algebras are dual Banach algebras. In this paper we find additional conditions under which the preduals $C_0(G)$ of $M(G)$ and $C^*(G)$ of $A(G)$ are uniquely determined. In both cases we consider a natural coassociative multiplication and show that the canonical predual gives rise to the unique weak$^*$-topology making both the multiplication separately weak$^*$-continuous and the coassociative multiplication weak$^*$-continuous. In particular, dual cohomological properties of these algebras are well defined with this additional structure.

math.FA↗
arXiv2008

Uncountable families of prime z-ideals in C_0(R)

Denote by $\continuum=2^{\aleph_0}$ the cardinal of continuum. We construct an intriguing family $(P_α: α\in\continuum)$ of prime $z$-ideals in $\C_0(\reals)$ with the following properties: If $f\in P_{i_0}$ for some $i_0\in\continuum$, then $f\in P_i$ for all but finitely many $i\in \continuum$; $\bigcap_{i\neq i_0} P_i \nsubset P_{i_0}$ for each $ı_0\in \continuum$. We also construct a well-ordered increasing chain, as well as a well-ordered decreasing chain, of order type $κ$ of prime $z$-ideals in $\C_0(\reals)$ for any ordinal $κ$ of cardinality $\continuum$.

math.RA↗