44 research outputs found
Bounds for Characters of the Symmetric Group: A Hypercontractive Approach
Finding upper bounds for character ratios is a fundamental problem in
asymptotic group theory. Previous bounds in the symmetric group have led to
remarkable applications in unexpected domains. The existing approaches
predominantly relied on algebraic methods, whereas our approach combines
analytic and algebraic tools. Specifically, we make use of a tool called
`hypercontractivity for global functions' from the theory of Boolean functions.
By establishing sharp upper bounds on the -norms of characters of the
symmetric group, we improve existing results on character ratios from the work
of Larsen and Shalev [Larsen M., Shalev A. Characters of the symmetric group:
sharp bounds and applications. Invent. math. 174 645-687 (2008)]. We use our
norm bounds to bound Kronecker coefficients, Fourier coefficients of class
functions, product mixing of normal sets, and mixing time of normal Cayley
graphs. Our approach bypasses the need for the -specific
Murnaghan--Nakayama rule. Instead we leverage more flexible representation
theoretic tools, such as Young's branching rule, which potentially extend the
applicability of our method to groups beyond
On the sum of the L1 influences of bounded functions
Let have degree as a multilinear
polynomial. It is well-known that the total influence of is at most .
Aaronson and Ambainis asked whether the total influence of can also
be bounded as a function of . Ba\v{c}kurs and Bavarian answered this
question in the affirmative, providing a bound of for general
functions and for homogeneous functions. We improve on their results
by providing a bound of for general functions and for
homogeneous functions. In addition, we prove a bound of for
monotone functions, and provide a matching example.Comment: 16 pages; accepted for publication in the Israel Journal of
Mathematic
Sharp Hypercontractivity for Global Functions
For a function on the hypercube with Fourier expansion
, the hypercontractive inequality for
the noise operator allows bounding norms of in terms of norms of . If is Boolean-valued, the level-
inequality allows bounding the norm of in
terms of . These two inequalities play a central role in analysis of
Boolean functions and its applications.
While both inequalities hold in a sharp form when the hypercube is endowed
with the uniform measure, they do not hold for more general discrete product
spaces. Finding a `natural' generalization was a long-standing open problem. In
[P. Keevash et al., Global hypercontractivity and its applications, J. Amer.
Math. Soc., to appear], a hypercontractive inequality for this setting was
presented, that holds for functions which are `global' -- namely, are not
significantly affected by a restriction of a small set of coordinates. This
hypercontractive inequality is not sharp, which precludes applications to the
symmetric group and to other settings where sharpness of the bound is
crucial. Also, no level- inequality for global functions over general
discrete product spaces is known.
We obtain sharp versions of the hypercontractive inequality and of the
level- inequality for this setting. Our inequalities open the way for
diverse applications to extremal set theory and to group theory. We demonstrate
this by proving quantitative bounds on the size of intersecting families of
sets and vectors under weak symmetry conditions and by describing numerous
applications to the study of functions on -- including hypercontractivity
and level- inequalities, character bounds, variants of Roth's theorem and of
Bogolyubov's lemma, and diameter bounds, that were obtained using our
techniques