Non-commutative standard polynomials applied to matrices
Abstract
The Amitsur–Levitski Theorem tells us that the standard polynomial in 2n non-commuting indeterminates vanishes identically over the matrix algebra M n (K). For K = R or C and 2 ≤ r ≤ 2n − 1, we investigate how big S r (A 1 ,. .. , A r) can be when A 1 ,. .. , A r belong to the unit ball. We privilegiate the Frobenius norm, for which the case r = 2 was solved recently by several authors. Our main result is a closed formula for the expectation of the square norm. We also describe the image of the unit ball when r = 2 or 3 and n = 2. MSC classification : 15A24, 15A27, 15A60
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