3,437 research outputs found
On binary correlations of multiplicative functions
We study logarithmically averaged binary correlations of bounded
multiplicative functions and . A breakthrough on these correlations
was made by Tao, who showed that the correlation average is negligibly small
whenever or does not pretend to be any twisted Dirichlet character,
in the sense of the pretentious distance for multiplicative functions. We
consider a wider class of real-valued multiplicative functions , namely
those that are uniformly distributed in arithmetic progressions to fixed
moduli. Under this assumption, we obtain a discorrelation estimate, showing
that the correlation of and is asymptotic to the product of their
mean values. We derive several applications, first showing that the number of
large prime factors of and are independent of each other with respect
to the logarithmic density. Secondly, we prove a logarithmic version of the
conjecture of Erd\H{o}s and Pomerance on two consecutive smooth numbers.
Thirdly, we show that if is cube-free and belongs to the Burgess regime
, the logarithmic average around of the real
character over the values of a reducible quadratic polynomial
is small.Comment: 33 pages; Referee comments incorporated; To appear in Forum Math.
Sigm
Congruence successions in compositions
A \emph{composition} is a sequence of positive integers, called \emph{parts},
having a fixed sum. By an \emph{-congruence succession}, we will mean a pair
of adjacent parts and within a composition such that . Here, we consider the problem of counting the compositions of
size according to the number of -congruence successions, extending
recent results concerning successions on subsets and permutations. A general
formula is obtained, which reduces in the limiting case to the known generating
function formula for the number of Carlitz compositions. Special attention is
paid to the case , where further enumerative results may be obtained by
means of combinatorial arguments. Finally, an asymptotic estimate is provided
for the number of compositions of size having no -congruence
successions
Evaluating Matrix Functions by Resummations on Graphs: the Method of Path-Sums
We introduce the method of path-sums which is a tool for exactly evaluating a
function of a discrete matrix with possibly non-commuting entries, based on the
closed-form resummation of infinite families of terms in the corresponding
Taylor series. If the matrix is finite, our approach yields the exact result in
a finite number of steps. We achieve this by combining a mapping between matrix
powers and walks on a weighted directed graph with a universal graph-theoretic
result on the structure of such walks. We present path-sum expressions for a
matrix raised to a complex power, the matrix exponential, matrix inverse, and
matrix logarithm. We show that the quasideterminants of a matrix can be
naturally formulated in terms of a path-sum, and present examples of the
application of the path-sum method. We show that obtaining the inversion height
of a matrix inverse and of quasideterminants is an NP-complete problem.Comment: 23 pages, light version submitted to SIAM Journal on Matrix Analysis
and Applications (SIMAX). A separate paper with the graph theoretic results
is available at: arXiv:1202.5523v1. Results for matrices over division rings
will be published separately as wel
The Bane of Low-Dimensionality Clustering
In this paper, we give a conditional lower bound of on
running time for the classic k-median and k-means clustering objectives (where
n is the size of the input), even in low-dimensional Euclidean space of
dimension four, assuming the Exponential Time Hypothesis (ETH). We also
consider k-median (and k-means) with penalties where each point need not be
assigned to a center, in which case it must pay a penalty, and extend our lower
bound to at least three-dimensional Euclidean space.
This stands in stark contrast to many other geometric problems such as the
traveling salesman problem, or computing an independent set of unit spheres.
While these problems benefit from the so-called (limited) blessing of
dimensionality, as they can be solved in time or
in d dimensions, our work shows that widely-used clustering
objectives have a lower bound of , even in dimension four.
We complete the picture by considering the two-dimensional case: we show that
there is no algorithm that solves the penalized version in time less than
, and provide a matching upper bound of .
The main tool we use to establish these lower bounds is the placement of
points on the moment curve, which takes its inspiration from constructions of
point sets yielding Delaunay complexes of high complexity
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