Write trace of AB* as summation (1 Viewer)

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i'm kinda confused regarding summation so i'm hoping someone can help me figure this out and explain to me why it is the way it is

trace(AB*) = ? in summation form

* = adjoint = conjugate and transpose = transpose and conjugate

assume both matrices are square mx of same size n x n

trace = sum of diagonal entries

i'm got this after brute force

(summation of this entire thing) a_ij x conjugate of (b_ij)

i, j runs from 1 to n


but somehow i'm thinking it should be

(summation of this entire thing) a_ii x conjugate of (b_ii)

i runs from 1 to n
 

Erland

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A diagonal element aii of a matrix product AB depends on all elements aik of row i in A and all elements bki of column i in B, not just on the diagonal elements of A and B. Therefore, the trace of AB must be the sum of aikbki over all i and k, not just the sum of aiibii.
 

Fredrik

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It sounds like what you're confused about isn't summation, but rather the definition of matrix multiplication. I'll quote myself:

I will denote the entry on row i, column j of an arbitrary matrix X by ##X_{ij}##. The definition of matrix multiplication says that if A is an m×n matrix, and B is an n×p matrix, then AB is the m×p matrix such that for all ##i\in\{1,\dots,m\}## and all ##j\in\{1,\dots,p\}##,
$$(AB)_{ij}=\sum_{k=1}^n A_{ik}B_{kj}.$$
The problem is quite easy if you just use the definitions. The other definitions you need to use are ##\operatorname{Tr X}=\sum_{i}X_{ii}## and ##(X^*)_{ij}=(X_{ji})^*##, where the first * denotes the adjoint operation and the second one denotes complex conjugation. But you don't seem to be confused about those.
 
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