# Complexity of Matrix Multiplication

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 :
* Is testing triangle-freeness as difficult as the Boolean multiplication of two |V| x |V | matrices?
A recent paper  addressess this problem partially. In a related note, the complexity of all pairs shortest paths (APSP) is still unresolved. Is APSP (for undirected graphs) as difficult as the Boolean multiplication of two |V| x |V | matrices?
 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.

Today’s post is about the following question and related problems.

Given an undirected graph $G(V,E)$, how fast can we detect if $G$ is triangle-free ?

Cubic time is obvious. A faster algorithm computes the square of the adjacency matrix of $G$ and runs in time $n^{\omega}$, where $\omega$ is the matrix multiplication exponent. This simple algorithm is the best known !!

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

Look at [AKKR’06] for recent results. A related problem is all pairs shortest paths (APSP). This is a fundamental graph problem and its exact complexity is still open.

Is APSP (for undirected graphs) as difficult as the Boolean multiplication of two |V| x |V | matrices?

These problems are tightly related to matrix multiplication, whose complexity is also open. The best value of $\omega$ is 2.376. At Barriers Workshop, Chris Umans presented an exciting group-theoretic approach [CU’03, CKSU’05] to improving $\omega$. Using their techniques, the best value of $\omega$ they achieved is 2.41. In their paper, they made two conjectures, one combinatorial and the other algebraic. Either one would imply that the exponent of matrix multiplication is 2.

Is $\omega= 2$ ?
Now, that would be a surprise in mathematics and theory. Many well-known problems are reducible to matrix multiplication. Eg : determinant, LUP decompositions, matrix inversion and many problems in linear algebra.

The techniques which led to 2.376 are primarily based on divide and conquer. The technique of [CU’03, CKSU’05] is a different one and worth pursuing. Are there any other techniques explored ? If you know of other techniques, please leave a comment, even if they resulted in a modest value of $\omega$.

References :

• [AKKR’06] 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.
• [CKSU’05] Henry Cohn, Robert D. Kleinberg, Balázs Szegedy, Christopher Umans. Group-theoretic Algorithms for Matrix Multiplication. FOCS 2005: 379-388
• [CU’03] Henry Cohn, Christopher Umans: A Group-Theoretic Approach to Fast Matrix Multiplication. FOCS 2003: 438-449