Trace of Alternating Product of Matrices and Almost Zero Matrices
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Theorem
Let $R$ be a ring with unity.
Let $n, m$ be positive integers.
Let $E_{ij}$ denote the $n \times n$ matrix with only zeroes except a $1$ at the $\tuple {i, j}$th element.
Let $A_1, \ldots, A_m \in R^{n \times n}$.
Let $i_k, j_k \in \set {1, \ldots, n}$ for $k \in \set {1, \ldots, m}$.
Let $i_0 = i_m$ and $j_0 = j_m$.
Then:
- $\map \tr {A_1 E_{i_1, j_1} A_2 E_{i_2, j_2} \cdots A_m E_{i_m, j_m} } = \ds \prod_{k \mathop = 1}^m \sqbrk {A_k}_{j_{k - 1} i_k}$
Proof
Use induction and the facts $E_{i j} A E_{k l} = A_{j k} E_{i l}$ and $\map \tr {A E_{i j} } = A_{j i}$ (induction basis).
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