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Iterative Methods for Sparse Linear Systems Second Edition

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2 CHAPTER 1. BACKGROUND IN LINEAR ALGEBRA<br />

• Multiplication by another matrix:<br />

C = AB,<br />

where A ∈ C n×m , B ∈ C m×p , C ∈ C n×p , and<br />

cij =<br />

m<br />

k=1<br />

aikbkj.<br />

Sometimes, a notation with column vectors and row vectors is used. The column<br />

vector a∗j is the vector consisting of the j-th column of A,<br />

a∗j =<br />

⎛<br />

⎜<br />

⎝<br />

a1j<br />

a2j<br />

.<br />

anj<br />

⎞<br />

⎟<br />

⎠ .<br />

Similarly, the notation ai∗ will denote the i-th row of the matrix A<br />

ai∗ = (ai1, ai2, . . .,aim).<br />

For example, the following could be written<br />

or<br />

A = (a∗1, a∗2, . . .,a∗m),<br />

⎛<br />

⎜<br />

A = ⎜<br />

⎝<br />

The transpose of a matrix A in C n×m is a matrix C in C m×n whose elements<br />

are defined by cij = aji, i = 1, . . .,m, j = 1, . . .,n. It is denoted by A T . It is often<br />

more relevant to use the transpose conjugate matrix denoted by A H and defined by<br />

a1∗<br />

a2∗<br />

.<br />

.<br />

an∗<br />

⎞<br />

⎟<br />

⎠ .<br />

A H = ĀT = A T ,<br />

in which the bar denotes the (element-wise) complex conjugation.<br />

Matrices are strongly related to linear mappings between vector spaces of finite<br />

dimension. This is because they represent these mappings with respect to two given<br />

bases: one <strong>for</strong> the initial vector space and the other <strong>for</strong> the image vector space, or<br />

range of A.

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