1. Coloring k-colorable graphs using smaller palettes
Eran Halperin Ram Nathaniel Uri Zwick
Tel Aviv University

2. New coloring results Coloring k-colorable graphs
of maximum degree D using
D1-2/k log1/kD colors
(instead of D1-2/k log1/2D colors [KMS])

3. New coloring results Coloring k-colorable graphs using
na(k) colors (instead of nb(k) colors [KMS])

4. An extension of Alon-Kahale
AK: If a graph contains an independent set of size n/k+m, k integer, then an independent set of size m3/(k+1) can be found in polynomial time.
Extension: If a graph contains an independent set of size n/a, then an independent set of size nf(a) can be found in polynomial time, where

5. Graph coloring basics
If in any k-colorable graph on n vertices we can find, in polynomial time, one of
Two vertices that have the same color under some valid k-coloring ;
An independent set of size W(n1-a) ;
then we can color any k-colorable graph using O(na) colors.

6. Coloring 3-colorable graphs using O(n1/2) colors [Wigderson]
A graph with maximum degree D can be easily colored using D+1 colors.
If D < n1/2, color using D+1 colors.
Otherwise, let v be a vertex of degree D.
Then, N(v) is 2-colorable and contains an independent set of size D/2>= n1/2/2.

7. Vector k-Coloring [KMS]
A vector k-coloring of a graph G=(V,E) is a sequence of unit vectors v1,v2,…,vn such that if (i,j) in E then <vi,vj>=-1/(k-1).

8. Finding large independent sets
Let G=(V,E) be a 3-colorable graph.
Let r be a random normally distributed vector in Rn. Let
I’ is obtained from I by removing a vertex from each edge of I.

9. Constructing the sets I and I’

10. Analysis

11. Analysis (Cont.)

13. Analysis (Cont.)

14. Analysis (Cont.)

15. A simple observation Suppose G=(V,E) is k-colorable.
Either G[N(u,v)] is (k-2)-colorable,
or u and v get the same color under
any a k-coloring of G.

16. A lemma of Blum Let G=(V,E) be a k-colorable graph with
minimum degree d
for every
Then, it is possible to construct, in polynomial time, a collection {Ti} of about n subsets of V such that at least one Ti satisfies:
|Ti|=W(d2/s)
Ti has an independent subset of size

17. A lemma of Blum

18. Graph coloring techniques
Karger Motwani Sudan
Wigderson
Blum
Alon
Kahale
Blum Karger
Our Algorithm

19. The new algorithm Step 0: If k=2, color the graph using 2 colors.
If k=3, color the graph using n3/14 colors using the algorithm of Blum and Karger.

20. The new algorithm Step 1:
Repeatedly remove from the graph vertices of degree at most na(k)/(1-2/k). Let U be the set of vertices removed, and W=V-U.
Average degree of G[U] is at most na(k)/(1-2/k).
Minimum degree of G[W] at least na(k)/(1-2/k).
If |U|>n/2, use [KMS] to find an independent set of size n/D1-2/k= n1-a(k).

21. Step 1 Let d=na(k)/(1-2/k). Average degree of G[U] is at most d.
Minimum degree of G[W] at least d.

22. The new algorithm Step 2:
For every u,v such that N(u,v)>n(1-a(k)/(1-a(k-2)),
apply the algorithm recursively on G[N(u,v)] and k-2.
If G[N(u,v)] is (k-2)-colorable, we get an independent set of size |N(u,v)|1-a(k-2)>n1-a(k).
Otherwise, we can infer* that u and v must be assigned the same color.

23. The new algorithm
Step 3: If we reach this step then |W|>n/2, the minimum degree of G[W] is at least na(k)/(1-2/k),
and for every u,v in W, N(u,v)>n(1-a(k)/(1-a(k-2)).
By Blum’s lemma, we can find a collection {Ti} of about n subsets of W such that at least one Ti satisfies |Ti|=W(d2/s) and Ti has an independent subset of size
By the extension of the Alon-Kahale result,
we can find an IS of size

24. The recurrence relation

25. [Guruswami,Hastad,Sudan ‘00]
Hardness results
It is NP-hard to 4-color 3-colorable graphs [Khanna,Linial,Safra ‘93] [Guruswami,Khanna ‘00]
For any k, it is NP-hard to k-color
2-colorable hypergraphs
[Guruswami,Hastad,Sudan ‘00]