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An equiangular tight frame (ETF) is a d×N matrix that has unit-norm columns and orthogonal rows of norm N/d. Its key property is that the absolute inner products between pairs of columns are (i) identical and (ii) as small as possible. ETFs have applications in communications, coding theory, and sparse approximation. Numerical experiments indicate that ETFs arise for very few pairs (d,N), and it is...
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