# Colloquium: Approximately Hadamard matrices and random frames

Friday, September 22, 2023 4pm

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38.94192000792223, -92.32805562883608

We will discuss a problem concerning random frames which arises in signal processing. A frame is an overcomplete set of vectors in the n-dimensional linear space which allows a robust decomposition of any vector in this space as a linear combination of these vectors. Random frames are used in signal processing as a means of encoding since the loss of a fraction of coordinates does not prevent the recovery. We will discuss a question when a random frame contains a copy of a nice (almost orthogonal) basis.

Despite the probabilistic nature of this problem it reduces to a completely deterministic question of existence of approximately Hadamard matrices. An n by n matrix with plus-minus 1 entries is called Hadamard if it acts on the space as a scaled isometry. Such matrices exist in some, but not in all dimensions. Nevertheless, we will construct plus-minus 1 matrices of every size which act as approximate scaled isometries. This construction will bring us back to probability as we will have to combine number-theoretic and probabilistic methods.

Joint work with Xiaoyu Dong.

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