Event Date: July 19, 2012 16:15

**The Johnson-Lindenstrauss Transform and Applications to Dimensionality Reduction**

The Johnson-Lindenstrauss transform is a fundamental dimensionality reduction technique with a wide range of applications in computer science. It is given by a projection matrix that maps vectors in Rˆd to Rˆk, where k << d, while seeking to approximately preserve their norm and pairwise distances. The classical result states that k = O(1/fˆ2 log 1/p) dimensions suffice to approximate the norm of any fixed vector in Rˆn to within a factor of 1 + f with probability at least 1-p, where 0 < p,f < 1. This is a remarkable result because the target dimension is independent of d. The projection matrix is itself produced by a random process that is oblivious to the input vectors. We show that the target dimension bound is optimal up to a constant factor, improving upon a previous result due to Noga Alon. This based on joint work with David Woodruff (SODA 2011).

BIO: Dr. T.S. Jayram is a manager in the Algorithms and Computation group at IBM Almaden Research Center and currently visiting IBM India Research Lab. He is interested in the theoretical foundations of massive data sets such as data streams, and has worked on both the algorithmic aspects and their limitations thereof. The latter has led to new techniques for proving lower bounds via the information complexity paradigm. For work in this area, he has received a Research Division Accomplishment Award in Science from IBM and was invited to give a survey talk on Information Complexity at PODS 2010.