## Tuesday, December 30, 2008

## Sunday, November 09, 2008

### The Good Will Hunting Problem

Here is a problem from the movie Good Will Hunting, shown in the screenshot below.

For the the graph G(V,E) shown above, find the following :

For the the graph G(V,E) shown above, find the following :

- The matrix giving the number of 3 step walks in G : [A
^{k}]_{ij}is the number of paths of length k from i to j. So, the answer is A^{3}.

- The generating function for walks from point i to j : The generating function is as follows. Here are more examples of generating functions.

- The generating function for walks from points 1 to 3 : Simplify the above formula using cramer's rule for i=1 and j=3.

## Tuesday, October 28, 2008

### Friendly Numbers

Pythagoras said "220 and 284 are friendly numbers" !! These numbers have a special property : Each is equal to the sum of the other's proper divisors. Proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110 (they sum to 284). Proper divisors of 284 are 1, 2, 4, 71 and 142 (they sum to 220). More example include (1184, 1210), (17296, 18416). It is not known whether there are infinitely many friendly numbers. Twin primes are a pair of consecutive odd numbers both of which are prime.

- Theorem : There are infinitely many friendly numbers. (proof)

- Conjecture : There are infinitely many twin primes.

## Wednesday, July 23, 2008

### Common Puzzles

Most of my friends are geeks. So when we get together on a friday night (or) driving to a conference, we don't talk about politics, movies or celebrities. Instead we throw math puzzles at each other. Here are some of the common puzzles I ran into....

The Banana-eating Camel : You have 3,000 bananas that must be transported across a desert that is 1,000 kilometers wide. You have a camel that has a 1,000 banana capacity. However, the camel must eat one banana for each kilometer that it walks. What is the largest number of bananas that can be transported across the desert?

- 12 Balls : There are 12 balls. They all look alike but one of them is faulty; it weights differently. It is not known, if this ball is heavier or lighter than the other balls. How to find the faulty ball by three weighs on a simple balance ?

- Linked Lists : You are given a pointer to the head of a singly linked list that might (or might not) have a loop somewhere (i.e., an element pointing back to an element). The length of the list is finite, but unknown. Devise an algorithm that detects if there is a loop. You must use only a constant amount of memory space and not destroy the list.

- Two Integers : I'm thinking of two integer numbers, each if them is more than 1 and their sum is less than 100. I tell my friend A the sum of these two numbers, and another friend B, product of these two numbers. Then such a dialog took place:

B: I can't determine what are these numbers.

A: Ah, i knew you wouldn't be able to do this.

B: Oh, then i know what they are!

A: Oh, then i know them too!

Can you determine the numbers?

- Stick triangle : A stick is broken at random into three pieces. What is the probability that the pieces can form a triangle?

## Thursday, April 24, 2008

### List Coloring of Planar Graphs

I have been reading some papers on list-coloring of planar graphs. Here's a quick overview of this topic.

A proper coloring of a graph is an assignment of colors to vertices of a graph such that no two adjacent vertices receive the same color. A graph is k-colorable if it can be properly colored with k colors. For example, the famous Four Color Theorem (4CT) states that "Evey planar graph is 4-colorable". This is tight, since K4 is 4-colorable but not 3-colorable. Deciding if a graph is 3-colorable is NP-hard. It is natural to ask which planar graphs are 3-colorable. Grotzsch's Theorem states that "Every triangle-tree planar graph is 3-colorable".

Given a graph and given a set L(v) of colors for each vertex v, a list coloring is a proper coloring such that every vertex v is assigned a color from the list L(v). A graph is k-list-colorable (or k-choosable) if it has a proper list coloring no matter how one assigns a list of k colors to each vertex.

If a graph is k-choosable then it is k-colorable (set each L(v) = {1,...k}). But the converse is not true. Following is a bipartite graph (2-colorable) that is not 2-choosable (corresponding lists are shown).

A graph is k-degenerate if each non-empty subgraph contains a vertex of degree at most k. The following fact is easy to prove by induction :

Note : A recent paper [DKT'08], presents a very short proof of Grotzsch's theorem and a linear-time algorithm for 3-coloring such graphs.

References :

A proper coloring of a graph is an assignment of colors to vertices of a graph such that no two adjacent vertices receive the same color. A graph is k-colorable if it can be properly colored with k colors. For example, the famous Four Color Theorem (4CT) states that "Evey planar graph is 4-colorable". This is tight, since K4 is 4-colorable but not 3-colorable. Deciding if a graph is 3-colorable is NP-hard. It is natural to ask which planar graphs are 3-colorable. Grotzsch's Theorem states that "Every triangle-tree planar graph is 3-colorable".

Given a graph and given a set L(v) of colors for each vertex v, a list coloring is a proper coloring such that every vertex v is assigned a color from the list L(v). A graph is k-list-colorable (or k-choosable) if it has a proper list coloring no matter how one assigns a list of k colors to each vertex.

If a graph is k-choosable then it is k-colorable (set each L(v) = {1,...k}). But the converse is not true. Following is a bipartite graph (2-colorable) that is not 2-choosable (corresponding lists are shown).

A graph is k-degenerate if each non-empty subgraph contains a vertex of degree at most k. The following fact is easy to prove by induction :

- A k-degenerate graph is (k+1)-choosable

- Every bipartite planar graph is 3-choosable [Alon & Tarsi]. It is easy to prove that every bipartite planar graph is 3-degenerate.
- Every planar is 5-choosable [Thomassen'94]. Note that every planar graph is 5-degenerate. There are planar graphs which are not 4-choosable [Voigt'93].
- Every planar graph of girth at least 5 is 3-choosable. This implies grotzsch's theorem in a very cute way [Thomassen'03]. There are planar graphs of girth 4 which are not 3-choosable [Voigt'95].

- Conjecture : Every 3-colorable planar graph is 4-choosable.

Note : A recent paper [DKT'08], presents a very short proof of Grotzsch's theorem and a linear-time algorithm for 3-coloring such graphs.

References :

- [Alon & Tarsi'92] N. Alon, M. Tarsi: Colorings and orientations of graphs. Combinatorica 12(2): 125-134 (1992)
- [Thomassen'94] C. Thomassen: Every Planar Graph Is 5-Choosable. J. Comb. Theory, Ser. B 62(1): 180-181 (1994)
- [Voigt'93] M. Voigt: List colourings of planar graphs. Discrete Mathematics 120(1-3): 215-219 (1993)
- [Thomassen'03] C. Thomassen: A short list color proof of Grötzsch's theorem. J. Comb. Theory, Ser. B 88(1): 189-192 (2003)
- [Voigt'95] M. Voigt : A not 3-choosable planar graph without 3-cycles. Discrete Mathematics 146(1-3): 325-328 (1995)
- [DKT'08] Z. Dvorak and K. Kawarabayashi and R. Thomas : Three-coloring triangle-free planar graphs in linear time. To appear in SODA 09.

## Sunday, April 20, 2008

### Testing triangle-freeness

Given an undirected graph G(V,E), how fast can we detect if G is triangle-free ? Cubic time is obvious. Let A be the adjacency matrix of G. We can detect triangle-freeness of G in the same complexity as multiplying two boolean matrices (AxA) (duh !!). This simple algorithm is the best known !! In other words, following is the open problem :

[1] N. Alon, T. Kaufman, M. Krivelevich, and D. Ron. Testing triangle-freeness in general graphs. Proceedings of the 17th Annual ACM-SIAM Symposium on Discrete Algorithms (SODA), pp. 279–-288, 2006.

- Is testing triangle-freeness as difficult as the Boolean multiplication of two |V| x |V | matrices?

[1] N. Alon, T. Kaufman, M. Krivelevich, and D. Ron. Testing triangle-freeness in general graphs. Proceedings of the 17th Annual ACM-SIAM Symposium on Discrete Algorithms (SODA), pp. 279–-288, 2006.

Labels:
graph theory,
open problems

## Friday, April 18, 2008

### Tiling chessboard by L-shaped trominoes

Can you cover all but one square of an n x n chessboard by L-shaped trominoes?

Claim : If n is a power of 2, you can always do it !!

Have fun proving this !!## Wednesday, April 09, 2008

### Lipton Symposium and Trotter Conference

I am eagerly waiting for the following two excellent conferences at GeorgiaTech :

- The Lipton Theory Symposium (Apr 26 - Apr 28 2008) : Celebrating Dick Lipton's 60th birthday. Consists of very diverse and interesting set of talks.

- New Directions in Algorithms, Combinatorics and Optimization (May 5 - May 9 2008) : Honoring the 65th Birthday of William T. Trotter. Excellent set of invited speakers and talks.

## Wednesday, April 02, 2008

### 25 Horses Puzzle

There are 25 horses and only five tracks in a race (i.e., you can race 5 horses at a time). There is no stop clock !! Assume that there are no ties.

1: What is the minimum number of races needed to determine the 3 fastest horses in order from fastest to slowest ?

2: ..... to find out the fastest one ?

3: ..... to rank all of them from fastest to slowest ?

4: ..... to find the top k fastest horses ?

1: What is the minimum number of races needed to determine the 3 fastest horses in order from fastest to slowest ?

2: ..... to find out the fastest one ?

3: ..... to rank all of them from fastest to slowest ?

4: ..... to find the top k fastest horses ?

## Wednesday, March 19, 2008

### Using Latex with Powerpoint

Next week, I am going to give a talk at DIMACS/DyDAn Workshop on Secure Internet Routing. While preparing slides for my presentation, I realized that I like powerpoint for its support for animation, but I hate using its equation-editor. Also, I don't like preparing slides in latex (using beamer) due to lack of decent animation tools. I was googling around for a solution and found the following alternatives to combine the best of both worlds :

1) TexPoint : I like its support for inline latex compilation. But it is not free and I found many limitations in math fonts and \displaystyle.

2) Tex4PPT : This does not support Office 2007 yet. So I did not explore it.

3) Inkscape : This is the BEST way to combine latex and powerpoint. Its is FREE and opensource too !! Install Inkscape and follow these instructions to add support for latex. Inkscape allows you to type any crazy latex equation and convert into .eps format. You can even ungroup symbols in an equation and assign different colors to different symbols. Add the .eps file in the ppt file (using Insert -> Picture) and you can zoom-in/zoom-out the image without sacrificing the resolution !!

1) TexPoint : I like its support for inline latex compilation. But it is not free and I found many limitations in math fonts and \displaystyle.

2) Tex4PPT : This does not support Office 2007 yet. So I did not explore it.

3) Inkscape : This is the BEST way to combine latex and powerpoint. Its is FREE and opensource too !! Install Inkscape and follow these instructions to add support for latex. Inkscape allows you to type any crazy latex equation and convert into .eps format. You can even ungroup symbols in an equation and assign different colors to different symbols. Add the .eps file in the ppt file (using Insert -> Picture) and you can zoom-in/zoom-out the image without sacrificing the resolution !!

Labels:
conferences,
miscellaneous

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