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The trace of a matrix A ∈ ℝ^(n×n), tr(A), is
AThe sum of all entries
BThe sum of the diagonal entries
CThe product of the eigenvalues, with sign
DThe largest singular value
Answer & Solution
Correct answer: B. The sum of the diagonal entries
tr(A) = Σ A_ii. It also equals the sum of eigenvalues (over the complex field), but its definition is the diagonal sum.
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