Thursday, April 28, 2005
| Title |
TBA |
| Speaker |
David Kephart |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Thursday, April 21, 2005
| Title |
On a vector space analogue of Kneser's theorem, Part II |
| Speaker |
Xiang-Dong Hou |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Thursday, April 14, 2005
| Title |
On a vector space analogue of Kneser's theorem |
| Speaker |
Xiang-Dong Hou |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
The following is a well known theorem in additive number
theory:
Theorem 1 (Kneser, 1953) Let G be an ableian group, written
multiplicatively, and let A, B be nonempty finite subsets of
G. Then |AB| ≥ |A| + |B| -
|H(AB)|, where AB = {ab: a
∈ A, b ∈ B} and
H(AB) = {g ∈ G: gAB
= AB} is the stabilizer of AB.
Recently, a vector space analogue of Kneser's theorem was found:
Theorem 2 (Hou, Leung, Xiang, 2002 JNT) Let
E ⊂ K be fields and let A, B
be E-subspace of K such that 0 < dim A <
∞, 0 < dim B < ∞. Assume that the algebraic closure of
E in K is separable over E. Then dim AB
≥ dim A + dim B - dim H(AB) where
AB is the E-space generated by {ab:a
∈ A, b ∈ B} and
H(AB) = {x ∈ K: xAB
= AB} is the stabilizer of AB in K.
In part I of this talk, we will review the proof Theorem 2. We conjecture that
Theorem 2 remains true without the separability assumption. In part II of this
talk, we will prove the conjecture with dim A ≤ 5.
Thursday, April 7, 2005
| Title |
Transitivity in Two-dimensional Languages |
| Speaker |
Joni Pirnot |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
In one-dimensional language theory, a language is said to be transitive if for
every pair of words in the language, there exists a sequence of symbols from the
alphabet that can ‘paste’ the two words together, forming another
(longer) word that appears in the language.
In two dimensions, the definition of transitivity is not so straightforward.
Dot systems are used to illustrate the need for an expanded notion of transitivity
when discussing two-dimensional languages.
Thursday, March 31, 2005
| Title |
Generating SAT solution spaces via splicing rules |
| Speaker |
Giuditta Franco |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
An ambition of DNA Computing is to solve NP-Complete problems in linear time.
The generation of the solution space is the first step of the classical method
for solving, in particular, an instance of SAT. For this problem, I will present
some combinatorial features of a (string) generation procedure based on null
context splicing rules. I will also discuss the generalization to the case
of non-boolean variables.
Thursday, March 24, 2005
| Title |
On Radical Deformations of Zero-dimensional Ideals |
| Speaker |
Boris Shekhtman |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
I will give a counterexample to a conjecture of Carl de Boor, showing that in
the ring of polynomials in three or more variables there exist zero-dimensional
ideals that do not admit radical deformations.
Time permitting, I will also propose an open problem that I think will be of
interest to this particular audience.
Thursday, March 10, 2005
| Title |
Surveillance and Forecasting for Waterborne Infections |
| Speaker |
Ian McNeill
Department of Statistical and Actuarial Sciences
University of Western Ontario
|
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
| Note |
This is a special session of the Complex Systems seminar. |
Abstract
A system is discussed for monitoring case count data on infections with a view
to early detection of outbreaks and to forecasting the extent of detected
outbreaks. The system is called INFERNO — a system for INtegrated Forecasts
and EaRly eNteric Outbreak detection. Historical data are smoothed using a
loess-type smoother. Upon receipt of a new datum, the smoothing is updated and
estimates are made of the first two derivatives of the smooth curve and these are
used for near-term forecasting. Recent data and the near-term forecasts are used
to compute a warning index. The algorithms for computing the warning index and the
interpretation of the index have been designed to effect a balance between Type I
errors (false prediction of an epidemic) and Type II errors (failure to correctly
predict an epidemic). If the warning index signals a sufficiently high probability
of an epidemic, then a forecast of the probable size of the outbreak is made. This
longer-term forecast is made by fitting a “signature” curve to the
available data. The effectiveness of the forecast depends upon the extent to which
the signature curve captures the shape of outbreaks of the infection under
consideration. Also, since the success of forecasting is partly determined by the
timeliness of the data, and since data are not usually available on a next-day
basis, a discussion is provided of a methodology of adjusting for reporting-delay.
Thursday, March 3, 2005
| Title |
A subgroup of the braid group associated with Fox's colorings |
| Speaker |
Shin Satoh |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
The set of braids with Fox's colorings, whose top and bottom strings have the
same vector of given colors, forms a subgroup of the braid group. We construct a
certain 2-complex from the vector, and prove that the subgroup is isomorphic to
the fundamental group of the 2-complex. In particular, we demonstrate the case of
3-string braids with 3-colorings.
Thursday, February 24, 2005
| Title |
Polynomial quandle cocycles and knot applications |
| Speaker |
Kheira Ameur |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
We construct new cocycles for Alexander quandles by polynomial expressions,
then we use them to compute quandle cocycle invariants for (2,m)-torus
knots and their twist spins.
Thursday, February 17, 2005
| Title |
On a Theorem of Schur |
| Speaker |
Peter Hilton
Distinguished Professor Emeritus
State University of New York, Binghamton |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
I. Schur proved that if G is a group, Z its center and
G' its commutator subgroup, and if G/Z is finite,
then G' is finite. Given a certain supplementary condition, the
converse also holds.
In this talk we prove the analog of Schur's Theorem and its converse in the
context of nilpotent groups. For such groups one can localize the problem at a
given family of primes P.
We also show how to relativize the theorem by replacing G by the
pair (G,N), where N is normal in G,
and replacing Z by CG (N),
the centralizer of N in G.
Thursday, February 10, 2005
| Title |
Divisibility properties of Fibonacci and Lucas numbers |
| Speaker |
Peter Hilton
Distinguished Professor Emeritus
State University of New York, Binghamton
|
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
We establish divisibility relations between Fibonacci and Lucas numbers. One
very striking result is the following: If the Fibonacci number
Fm divides some Lucas number
Ln, then Fm =
1, 2 or 3. Another surprising result is that if Ln
divides Fm, then
Fn divides Fm
and yet Ln and
Fn are “almost” coprime.
Thursday, February 3, 2005
| Title |
Logical Operators III: the Transitive Closure Operator |
| Speaker |
Professor Greg McColm |
| Time |
4:00-5:00 p.m. |
| Place |
LIF 269 |
Abstract
This is the third of three talks aimed at describing lattice-valued (e.g.,
boolean and/or fuzzy) Least Fixed Point logic and the Transitive Closure operator,
a logical operator intimately connected with NLOGSPACE.
In this talk, we present the Transitive Closure Operator, and a game-theoretic
representation of it.
Thursday, January 27, 2005
| Title |
Logical Operators II: Game Semantics |
| Speaker |
Professor Greg McColm |
| Time |
4:00-5:00 p.m. |
| Place |
ENC 1002 |
Abstract
This is the second of several talks aimed at describing lattice-valued (e.g.,
boolean and/or fuzzy) Least Fixed Point logic and the Transitive Closure operator,
a logical operator intimately connected with NLOGSPACE.
In this talk, we present a game-theoretic logic based on operators on a lattice
of game-value functions.
Thursday, January 20, 2005
| Title |
Logical Operators I: Fixed Points |
| Speaker |
Professor Greg McColm |
| Time |
4:00-5:00 p.m. |
| Place |
NES 103 |
Abstract
This is the first of several talks aimed at describing the Transitive Closure
operator, a logical operator intimately connected with NLOGSPACE.
In this talk, we start at the beginning, with a description of operators on
lattices, the existence and construction of fixed points of operators, and the
consequent extension of First Order logic.