Theoretical computer science day in Sydney

January 28, 2011

New Law School Seminar 020, University of Sydney

CALL FOR PARTICIPATION

The theoretical computer science day will take place in Sydney on

January 28, 2011. The aim of this one-day event is to bring together

researchers interested in all aspects of theoretical computer science,

and discuss current trends and problems in the field, as well as

future directions. We would like to invite researchers in theoretical

computer science to participate in this event. If you are interested

in participating, please register as soon as possible for the event

through the website mentioned below. Registration is open to everyone

and is free.

More information is available on the web at

http://sact.it.usyd.edu.au/TDS, including information about the

invited presentations, the location and registration.

The list of invited speakers includes:

Otfried Cheong, KAIST, Korea

Catherine Greenhill, UNSW

Mohammad Mahdian, Yahoo! research

Bernard Mans, Macquarie University

Toby Walsh, NICTA

David Wood, University of Melbourne

ORGANISATION

Joachim Gudmundsson, NICTA, joachim.gudmundsson@nicta.com.au

Julian Mestre, University of Sydney, mestre@it.usyd.edu.au

Taso Viglas, University of Sydney, taso.viglas@sydney.edu.au

## Thursday, December 9, 2010

## Tuesday, November 30, 2010

### Another great JoCG article

Posted by
Joachim
at
6:04 PM

Computing the Maximum Detour of a Plane Geometric Graph in Subquadratic Time

by Christian Wulff-Nilsen

Abstract:

Let G be a plane geometric graph where each edge is a line segment. We consider the problem of computing the maximum detour of G, defined as the maximum over all pairs of distinct points (vertices as well as interior points of edges) p and q of G of the ratio between the distance between p and q in G and the Euclidean distance ||pq||2. The fastest known algorithm for this problem has Θ(n2) running time where n is the number of vertices. We show how to obtain O(n3/2(log n)3) expected running time. We also show that if G has bounded treewidth, its maximum detour can be computed in O(n(log n)3) expected time.

by Christian Wulff-Nilsen

Abstract:

Let G be a plane geometric graph where each edge is a line segment. We consider the problem of computing the maximum detour of G, defined as the maximum over all pairs of distinct points (vertices as well as interior points of edges) p and q of G of the ratio between the distance between p and q in G and the Euclidean distance ||pq||2. The fastest known algorithm for this problem has Θ(n2) running time where n is the number of vertices. We show how to obtain O(n3/2(log n)3) expected running time. We also show that if G has bounded treewidth, its maximum detour can be computed in O(n(log n)3) expected time.

## Sunday, November 21, 2010

### JoCG paper

Posted by
Joachim
at
7:43 PM

New paper published in JoCG:

COMPUTING MULTIDIMENSIONAL PERSISTENCE

Gunnar Carlsson, Gurjeet Singh, Afra J. Zomorodian

ABSTRACT

The theory of multidimensional persistence captures the topology of a multifiltration - a multiparameter family of increasing spaces. Multifiltrations arise naturally in the topological analysis of scientific data. In this paper, we give a polynomial time algorithm for computing multidimensional persistence. We recast this computation as a problem within computational commutative algebra and utilize algorithms from this area to solve it. While the resulting problem is EXPSPACE-complete and the standard algorithms take doubly-exponential time, we exploit the structure inherent withing multifiltrations to yield practical algorithms. We implement all algorithms in the paper and provide statistical experiments to demonstrate their feasibility.

COMPUTING MULTIDIMENSIONAL PERSISTENCE

Gunnar Carlsson, Gurjeet Singh, Afra J. Zomorodian

ABSTRACT

The theory of multidimensional persistence captures the topology of a multifiltration - a multiparameter family of increasing spaces. Multifiltrations arise naturally in the topological analysis of scientific data. In this paper, we give a polynomial time algorithm for computing multidimensional persistence. We recast this computation as a problem within computational commutative algebra and utilize algorithms from this area to solve it. While the resulting problem is EXPSPACE-complete and the standard algorithms take doubly-exponential time, we exploit the structure inherent withing multifiltrations to yield practical algorithms. We implement all algorithms in the paper and provide statistical experiments to demonstrate their feasibility.

## Tuesday, October 19, 2010

### Call for papers!

Posted by
Joachim
at
11:57 AM

Some recent CG related cfp:

- Symposium on Computational Geometry (SoCG'11) will be held on June 13-15 in Paris, France. Deadline: Dec 1

There is also a CG Learning Summer School (June 9-11) as a satellite event.

- European Workshop on Computational Geometry (EuroCG'11) will be held on March 28-30, 2011, in Morschach, Switzerland. Deadline: Jan 9

- Conference on Geosensor Networks (GSN'11) will be held on July 11-13, 2011, in Melbourne, Australia. Deadline: Mar 4

- Symposium on Computational Geometry (SoCG'11) will be held on June 13-15 in Paris, France. Deadline: Dec 1

There is also a CG Learning Summer School (June 9-11) as a satellite event.

- European Workshop on Computational Geometry (EuroCG'11) will be held on March 28-30, 2011, in Morschach, Switzerland. Deadline: Jan 9

- Conference on Geosensor Networks (GSN'11) will be held on July 11-13, 2011, in Melbourne, Australia. Deadline: Mar 4

## Wednesday, June 30, 2010

### Some SoCG'10 papers

Posted by
Joachim
at
7:58 PM

SoCG is over for this year. In general I thought that the quality of the accepted papers was very high, and I really enjoyed many of the presentations and the results. There has already been numerous reports from SoCG (Sorelle I, II and III, David, Sariel and Suresh), and below I'm just going to briefly mention three of the ones that I found the most interesting - in order of presentation.

Timothy Chan - Optimal Partition Trees (Paper).

Timothy improves on a result from 1992 by Matousek. He presents a simpler construction with a better preprocessing time. There are already numerous blog comments on it (Sariel and Sorelle).

Joe Mitchell - A constant factor approximation algorithm for TSP with neighbourhoods in the plane (paper).

This is a problem I've been working on back and forth for a long time. Given a collection of objects (usually polygons) in the plane, called neighbourhoods, find the shortest tour that visits all neighbourhoods. In 2002 we presented a constant factor approximation algorithm for fat disjoint objects of arbitrary size. This was the first constant factor approximation algorithm for arbitrary size objects. Joe has several results on the topic and in his SoCG paper he presents the first constant-factor approximation algorithm for TSPN on an arbitrary set of disjoint, connected neighbourhoods in the plane. This has been an open problem since the mid-90's and Joe's paper is a big step forward. Of course, ultimately we would like a constant-factor approximation for the general case, not necessarily disjoint neighbourhoods. He has several interesting general observations in his paper. One is that skinny neighbourhoods are not longer a big problem. Using “directional hulls”, which are fat, for an appropriate choice of one of two coordinate systems. Then one can partition the regions into two sets (“blue” regions and “red” regions). Separately find approximating tours of each set, and then use the Combination Lemma (Arkin and Hassin) to combine into a tour of the entire set.

Anne Driemel, Sariel Har-Peled and Carola Wenk - Approximating the Frechet distance for realistic curves in near linear time (paper).

This is probably my favourite paper at SoCG this year. It's an important problem (imho) and it's great to finally see a subquadratic time algorithm with reasonable assumptions. The aim is to calculate the Frechet distance between two curves. They introduce a notion of k-bounded curvature which means that in a ball of radius r the length of the trajectory inside that ball is bounded by k*r (this family of curves is closed under simplification, a property that is very useful in practice). This sounds like a reasonable assumption for many applications (although not for trajectories generated from football players). They present a (1+eps)-approximation algorithm to compute the Frechet distance between two curves with k-bounded curvature and prove that this can be done in near-linear time. The key idea is that after the curves have been simplified the complexity of the reachable free space is linear for fixed k and epsilon (the free space diagram is the standard tool for computing the Frechet distance between curves but has quadratic complexity).

Timothy Chan - Optimal Partition Trees (Paper).

Timothy improves on a result from 1992 by Matousek. He presents a simpler construction with a better preprocessing time. There are already numerous blog comments on it (Sariel and Sorelle).

Joe Mitchell - A constant factor approximation algorithm for TSP with neighbourhoods in the plane (paper).

This is a problem I've been working on back and forth for a long time. Given a collection of objects (usually polygons) in the plane, called neighbourhoods, find the shortest tour that visits all neighbourhoods. In 2002 we presented a constant factor approximation algorithm for fat disjoint objects of arbitrary size. This was the first constant factor approximation algorithm for arbitrary size objects. Joe has several results on the topic and in his SoCG paper he presents the first constant-factor approximation algorithm for TSPN on an arbitrary set of disjoint, connected neighbourhoods in the plane. This has been an open problem since the mid-90's and Joe's paper is a big step forward. Of course, ultimately we would like a constant-factor approximation for the general case, not necessarily disjoint neighbourhoods. He has several interesting general observations in his paper. One is that skinny neighbourhoods are not longer a big problem. Using “directional hulls”, which are fat, for an appropriate choice of one of two coordinate systems. Then one can partition the regions into two sets (“blue” regions and “red” regions). Separately find approximating tours of each set, and then use the Combination Lemma (Arkin and Hassin) to combine into a tour of the entire set.

Anne Driemel, Sariel Har-Peled and Carola Wenk - Approximating the Frechet distance for realistic curves in near linear time (paper).

This is probably my favourite paper at SoCG this year. It's an important problem (imho) and it's great to finally see a subquadratic time algorithm with reasonable assumptions. The aim is to calculate the Frechet distance between two curves. They introduce a notion of k-bounded curvature which means that in a ball of radius r the length of the trajectory inside that ball is bounded by k*r (this family of curves is closed under simplification, a property that is very useful in practice). This sounds like a reasonable assumption for many applications (although not for trajectories generated from football players). They present a (1+eps)-approximation algorithm to compute the Frechet distance between two curves with k-bounded curvature and prove that this can be done in near-linear time. The key idea is that after the curves have been simplified the complexity of the reachable free space is linear for fixed k and epsilon (the free space diagram is the standard tool for computing the Frechet distance between curves but has quadratic complexity).

## Tuesday, June 29, 2010

### New position at the university of sydney (algorithmic machine learning)

Posted by
taso viglas
at
9:17 PM

We have a new position in "algorithmic machine learning" at the university of sydney.

The deadline for submitting an application is 12-august-2010.

The position is at the level of "lecturer" which is the same as an assistant professor, or "senior lecturer" which is again the same as an assistant professor but usually slightly older, with some gray hair, possibly kids, and a mortgage.

The deadline for submitting an application is 12-august-2010.

The position is at the level of "lecturer" which is the same as an assistant professor, or "senior lecturer" which is again the same as an assistant professor but usually slightly older, with some gray hair, possibly kids, and a mortgage.

## Monday, June 14, 2010

### More JoCG articles

Posted by
Joachim
at
2:56 AM

Another two articles published in JoCG (and currently another 16 under review):

On the Stretch Factor of Convex Delaunay Graphs

by Prosenjit Bose, Paz Carmi, Sebastien Collette and Michiel Smid

Visibility Maps of Realistic Terrains have Linear Smoothed Complexity

by Mark de Berg, Herman Haverkort and Constantinos P. Tsirogiannis

Just arrived in Snowbird for SoCG'10. I was hoping for a warm summer weather, instead it's raining and there's still snow around the hotel.

On the Stretch Factor of Convex Delaunay Graphs

by Prosenjit Bose, Paz Carmi, Sebastien Collette and Michiel Smid

Visibility Maps of Realistic Terrains have Linear Smoothed Complexity

by Mark de Berg, Herman Haverkort and Constantinos P. Tsirogiannis

Just arrived in Snowbird for SoCG'10. I was hoping for a warm summer weather, instead it's raining and there's still snow around the hotel.

## Tuesday, June 8, 2010

### JoCG new article

Posted by
Joachim
at
10:45 PM

The second scientific article in JoCG was just published. It's an expository of visibility and blockers by Attila Por and David Wood. Enjoy!

## Tuesday, May 25, 2010

### CATS 2011 call for papers (the australian theory conference)

Posted by
taso viglas
at
12:01 PM

CATS 2011 -- Computing: The Australasian Theory Symposium

Perth, Australia, January 17-20, 2011

http://cats.it.usyd.edu.au/

The 17th Computing: The Australasian Theory Symposium (CATS) will be held in Perth, Australia, January 17-20, 2011. CATS is an annual conference held in the Australia-New Zealand region, dedicated to theoretical computer science.

Authors are invited to submit papers that present original and unpublished research on topics including (but not limited to) the following areas: algorithms and data structures, complexity theory, graph theory, graph algorithms and combinatorics, semantics of programming languages, approximation and randomized algorithms, combinatorial optimization, formal program specification and transformation, computational geometry, algorithmic game theory, computational biology, logic and type systems, and computability.

Deadlines and other dates:

Paper submission deadline:Monday August 16, 2010

Acceptance notification:Monday October 4, 2010

Final version of accepted papers due:Monday November 5, 2010

Early registration: Monday December 6, 2010

Conference dates: January 17-20, 2011

The proceedings of this event will be published by the Australian Computer Society (ACS) in the CRPIT Series (http://crpit.com/), and will also appear in the ACM digital library.

CATS 2011 is part of the Australasian Computer Science Week (ACSW), an international annual conference event, supported by the Computing Research and Education Association (CORE) in Australia. ACSW 2011 is hosted by Curtin University in Perth, Australia.

For more information please visit http://cats.it.usyd.edu.au/

Contact: taso viglas (aviglas+cats2011@gmail.com)

Perth, Australia, January 17-20, 2011

http://cats.it.usyd.edu.au/

The 17th Computing: The Australasian Theory Symposium (CATS) will be held in Perth, Australia, January 17-20, 2011. CATS is an annual conference held in the Australia-New Zealand region, dedicated to theoretical computer science.

Authors are invited to submit papers that present original and unpublished research on topics including (but not limited to) the following areas: algorithms and data structures, complexity theory, graph theory, graph algorithms and combinatorics, semantics of programming languages, approximation and randomized algorithms, combinatorial optimization, formal program specification and transformation, computational geometry, algorithmic game theory, computational biology, logic and type systems, and computability.

Deadlines and other dates:

Paper submission deadline:Monday August 16, 2010

Acceptance notification:Monday October 4, 2010

Final version of accepted papers due:Monday November 5, 2010

Early registration: Monday December 6, 2010

Conference dates: January 17-20, 2011

The proceedings of this event will be published by the Australian Computer Society (ACS) in the CRPIT Series (http://crpit.com/), and will also appear in the ACM digital library.

CATS 2011 is part of the Australasian Computer Science Week (ACSW), an international annual conference event, supported by the Computing Research and Education Association (CORE) in Australia. ACSW 2011 is hosted by Curtin University in Perth, Australia.

For more information please visit http://cats.it.usyd.edu.au/

Contact: taso viglas (aviglas+cats2011@gmail.com)

## Tuesday, May 11, 2010

### Back in the office

Posted by
Joachim
at
5:02 PM

Today is my first day back at work after three months daddy leave. It's been great (I highly recommend it!) but it's also good to be back. I feel pretty energetic and motivated, but I'm sure it's just temporary.

As part of the new start I refurbished my homepage (can you guess what the banner depicts?), I bought two new sweaters and I just printed (I really should get an ereader) 6 papers that I have to review. Yes, I'm ready for work!

As part of the new start I refurbished my homepage (can you guess what the banner depicts?), I bought two new sweaters and I just printed (I really should get an ereader) 6 papers that I have to review. Yes, I'm ready for work!

## Monday, February 15, 2010

### SoCG paper

Posted by
Joachim
at
11:08 AM

As Sariel already noted the SoCG notification is out (couldn't find a list of accepted papers).

I was lucky to get a paper accepted. The paper is on visibility testing in the plane and it's together with Pat Morin.

In the paper we consider query versions of visibility testing and visibility counting. That is, let S be a set of n disjoint line segments in the plane and let s be an element of S. Visibility testing is to preprocess S so that we can quickly determine if s is visible from a query point q. The counting version involves preprocessing S so that one can quickly estimate the number of segments in S visible from a query point q.

The key idea is that one can cover the visibility polygon of a segment with only an expected linear number of triangles (even though its complexity may be quartic), and for all the segments with a quadratic number of triangles, such that given a query point q the number of triangles stabbed is a 2-approximation of the number of segments q can see. Pretty nifty, eh?

I was lucky to get a paper accepted. The paper is on visibility testing in the plane and it's together with Pat Morin.

In the paper we consider query versions of visibility testing and visibility counting. That is, let S be a set of n disjoint line segments in the plane and let s be an element of S. Visibility testing is to preprocess S so that we can quickly determine if s is visible from a query point q. The counting version involves preprocessing S so that one can quickly estimate the number of segments in S visible from a query point q.

The key idea is that one can cover the visibility polygon of a segment with only an expected linear number of triangles (even though its complexity may be quartic), and for all the segments with a quadratic number of triangles, such that given a query point q the number of triangles stabbed is a 2-approximation of the number of segments q can see. Pretty nifty, eh?

## Friday, February 12, 2010

### JoCG first publications!

Posted by
Joachim
at
9:34 AM

I'm sure some of you are wondering what's happening with JoCG (Journal of Computational Geometry). The first papers were published yesterday!

The first paper of JoCG is a Welcome from the Editors-in-Chief by Ken Clarkson and Günter Rote. And the first contribution to be published is: Happy Endings for Flip Graphs by David Eppstein.

So far JoCG has received 21 submissions. Of these, 1 has been accepted and published, 4 have been accepted pending (various levels of) revisions, 8 have been rejected, and the remaining 8 papers are still being reviewed.

All in all, I think it's a good start of the journal. Hopefully it will get even more submissions now when papers are getting published.

The first paper of JoCG is a Welcome from the Editors-in-Chief by Ken Clarkson and Günter Rote. And the first contribution to be published is: Happy Endings for Flip Graphs by David Eppstein.

So far JoCG has received 21 submissions. Of these, 1 has been accepted and published, 4 have been accepted pending (various levels of) revisions, 8 have been rejected, and the remaining 8 papers are still being reviewed.

All in all, I think it's a good start of the journal. Hopefully it will get even more submissions now when papers are getting published.

## Wednesday, January 20, 2010

### Computational geometry - main stream convex optimisation?

Posted by
Joachim
at
5:42 PM

As Tasos pointed out there's an open position in "Large Scale Convex Optimization" at University of Sydney.

For that reason I would be very interested to hear if anyone has any ideas on how one can argue that computational geometry is actually "Large Scale Convex Optimization", or even that it's remotely related to it.

Really good ideas will be awarded with beer (non-alcoholic of course).

For that reason I would be very interested to hear if anyone has any ideas on how one can argue that computational geometry is actually "Large Scale Convex Optimization", or even that it's remotely related to it.

Really good ideas will be awarded with beer (non-alcoholic of course).

## Friday, January 15, 2010

### Position at the University of Sydney

Posted by
taso viglas
at
11:53 AM

There is a faculty position at the University of Sydney (School of IT). The full ad can be found here.

The position is in the area of optimization and algorithms.

Applications need to be submitted by January 27. The position is at the Lecturer/Senior Lecturer level which is what an assistant professor is called in australia (senior lecturer can also be considered as an early associate professor level appointment).

--taso

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