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Single-Particle Electrodynamics - Assassination Science

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To write the result (A.39) in component-free notation, we must relabel the<br />

indices in the second term via j ↔ k, and make use of the identity ε ikj ≡<br />

−ε ijk ; thus,<br />

{A×B } ≡ 1 )<br />

A×B − B×A . (A.40)<br />

2(<br />

Conceptually, the minus sign in (A.40) may be understood by considering<br />

the case when the factors in the symmetrisor are c-numbers, in which case<br />

the various permuted terms must of course be identical: for c-number three<br />

vectors A and B, we note the permutation identity A × B ≡ −B × A, and<br />

hence (A.40) is the correct identity.<br />

A.7 Matrices<br />

A matrix is, for the purposes of this thesis, a collection object which is<br />

conveniently arranged as a regular rectangular array of quantities.<br />

Any mathematical quantity that is, for the purposes of the investigation<br />

in question, considered to be represented in matrix form is referred to as a<br />

matricised quantity. Matricised quantities are denoted by symbols from the<br />

standard symbol set.<br />

A.7.1<br />

Rows and columns<br />

Matrices are considered to be constructed of rows and columns; there is a<br />

matrix element for the conceptual intersection of each row and column.<br />

The number of rows and number of columns possessed by any given matrix<br />

are together referred to as the dimensions of the matrix; the dimensions are<br />

denoted N rows × N columns , where N rows is the number of rows and N columns is<br />

the number of columns.<br />

343

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