160 research outputs found
Determinants of grids, tori, cylinders and M\"{o}bius ladders
Recently, Bie\~{n} [A. Bie\~{n}, The problem of singularity for planar grids,
Discrete Math. 311 (2011), 921--931] obtained a recursive formula for the
determinant of a grid. Also, recently, Pragel [D. Pragel, Determinants of box
products of paths, Discrete Math. 312 (2012), 1844--1847], independently,
obtained an explicit formula for this determinant. In this paper, we give a
short proof for this problem. Furthermore, applying the same technique, we get
explicit formulas for the determinant of a torus, a cylinder, and a M\"{o}bius
ladder
On fully split lacunary polynomials in finite fields
We estimate the number of possible types degree patterns of -lacunary
polynomials of degree which split completely modulo . The result is
based on a combination of a bound on the number of zeros of lacunary
polynomials with some graph theory arguments.Comment: 8 pages. Bull. Polish Acad. Sci. Math., to appea
Counting surface-kernel epimorphisms from a co-compact Fuchsian group to a cyclic group with motivations from string theory and QFT
Graphs embedded into surfaces have many important applications, in
particular, in combinatorics, geometry, and physics. For example, ribbon graphs
and their counting is of great interest in string theory and quantum field
theory (QFT). Recently, Koch, Ramgoolam, and Wen [Nuclear Phys.\,B {\bf 870}
(2013), 530--581] gave a refined formula for counting ribbon graphs and
discussed its applications to several physics problems. An important factor in
this formula is the number of surface-kernel epimorphisms from a co-compact
Fuchsian group to a cyclic group. The aim of this paper is to give an explicit
and practical formula for the number of such epimorphisms. As a consequence, we
obtain an `equivalent' form of the famous Harvey's theorem on the cyclic groups
of automorphisms of compact Riemann surfaces. Our main tool is an explicit
formula for the number of solutions of restricted linear congruence recently
proved by Bibak et al. using properties of Ramanujan sums and of the finite
Fourier transform of arithmetic functions
Improving the smoothed complexity of FLIP for max cut problems
Finding locally optimal solutions for max-cut and max--cut are well-known
PLS-complete problems. An instinctive approach to finding such a locally
optimum solution is the FLIP method. Even though FLIP requires exponential time
in worst-case instances, it tends to terminate quickly in practical instances.
To explain this discrepancy, the run-time of FLIP has been studied in the
smoothed complexity framework. Etscheid and R\"{o}glin showed that the smoothed
complexity of FLIP for max-cut in arbitrary graphs is quasi-polynomial. Angel,
Bubeck, Peres, and Wei showed that the smoothed complexity of FLIP for max-cut
in complete graphs is , where is an upper bound on
the random edge-weight density and is the number of vertices in the input
graph.
While Angel et al.'s result showed the first polynomial smoothed complexity,
they also conjectured that their run-time bound is far from optimal. In this
work, we make substantial progress towards improving the run-time bound. We
prove that the smoothed complexity of FLIP in complete graphs is . Our results are based on a carefully chosen matrix whose rank
captures the run-time of the method along with improved rank bounds for this
matrix and an improved union bound based on this matrix. In addition, our
techniques provide a general framework for analyzing FLIP in the smoothed
framework. We illustrate this general framework by showing that the smoothed
complexity of FLIP for max--cut in complete graphs is polynomial and for
max--cut in arbitrary graphs is quasi-polynomial. We believe that our
techniques should also be of interest towards addressing the smoothed
complexity of FLIP for max--cut in complete graphs for larger constants .Comment: 36 page
On an almost-universal hash function family with applications to authentication and secrecy codes
Universal hashing, discovered by Carter and Wegman in 1979, has many
important applications in computer science. MMH, which was shown to be
-universal by Halevi and Krawczyk in 1997, is a well-known universal
hash function family. We introduce a variant of MMH, that we call GRDH,
where we use an arbitrary integer instead of prime and let the keys
satisfy the
conditions (), where are
given positive divisors of . Then via connecting the universal hashing
problem to the number of solutions of restricted linear congruences, we prove
that the family GRDH is an -almost--universal family of
hash functions for some if and only if is odd and
. Furthermore, if these conditions are
satisfied then GRDH is -almost--universal, where is
the smallest prime divisor of . Finally, as an application of our results,
we propose an authentication code with secrecy scheme which strongly
generalizes the scheme studied by Alomair et al. [{\it J. Math. Cryptol.} {\bf
4} (2010), 121--148], and [{\it J.UCS} {\bf 15} (2009), 2937--2956].Comment: International Journal of Foundations of Computer Science, to appea
Restricted linear congruences
In this paper, using properties of Ramanujan sums and of the discrete Fourier
transform of arithmetic functions, we give an explicit formula for the number
of solutions of the linear congruence ,
with (), where
() are arbitrary integers. As a consequence, we derive necessary and
sufficient conditions under which the above restricted linear congruence has no
solutions. The number of solutions of this kind of congruence was first
considered by Rademacher in 1925 and Brauer in 1926, in the special case of
. Since then, this problem has been studied, in
several other special cases, in many papers; in particular, Jacobson and
Williams [{\it Duke Math. J.} {\bf 39} (1972), 521--527] gave a nice explicit
formula for the number of such solutions when . The problem is very well-motivated and has found intriguing
applications in several areas of mathematics, computer science, and physics,
and there is promise for more applications/implications in these or other
directions.Comment: Journal of Number Theory, to appea
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