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Mathematical Methods for Physicists: A concise introduction - Site Map

Mathematical Methods for Physicists: A concise introduction - Site Map

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LINEAR VECTOR SPACES<br />

Eq. (5.16) determines the vector x 0 if the vector x is given, and …† ~ is the operator<br />

(matrix representation of the rotation operator) which turns x into x 0 .<br />

Loosely speaking, an operator is any mathematical entity which operates on<br />

any vector in V and turns it into another vector in V. Abstractly, an operator L is<br />

~<br />

a mapping that assigns to a vector jvi in a linear vector space V another vector jui<br />

in V: jui ˆL jvi. The set of vectors jvi <strong>for</strong> which the mapping is de®ned, that is,<br />

~<br />

the set of vectors jvi <strong>for</strong> which L jvi has meaning, is called the domain of L . The<br />

~ ~<br />

set of vectors jui in the domain expressible as jui ˆL jvi is called the range of the<br />

~<br />

operator. An operator L is linear if the mapping is such that <strong>for</strong> any vectors<br />

~<br />

jui; jwi in the domain of L and <strong>for</strong> arbitrary scalars , , the vector<br />

~<br />

jui‡jwi is in the domain of L and<br />

~<br />

L<br />

~<br />

…jui‡jwi† ˆ L<br />

~<br />

jui‡L<br />

~<br />

jwi:<br />

A linear operator is bounded if its domain is the entire space V and if there exists a<br />

single constant C such that<br />

jL<br />

~<br />

jvij < Cjjvij<br />

<strong>for</strong> all jvi in V. We shall consider linear bounded operators only.<br />

Matrix representation of operators<br />

Linear bounded operators may be represented by matrix. The matrix will have a<br />

®nite or an in®nite number of rows according to whether the dimension of V is<br />

®nite or in®nite. To show this, let j1i; j2i; ... be an orthonormal basis in V; then<br />

every vector j'i in V may be written in the <strong>for</strong>m<br />

Since L<br />

~<br />

ji is also in V, we may write<br />

But<br />

so<br />

j'i ˆ 1 j1i‡ 2 j2i‡:<br />

L<br />

~<br />

j'i ˆ 1 j1i‡ 2 j2i‡:<br />

L<br />

~<br />

j'i ˆ 1 L<br />

~<br />

j1i‡ 2 L<br />

~<br />

j2i‡;<br />

1 j1i‡ 2 j2i‡ˆ 1 L<br />

~<br />

j1i‡ 2 L<br />

~<br />

j2i‡:<br />

Taking the inner product of both sides with h1j we obtain<br />

1 ˆh1jL<br />

~<br />

j1i 1 ‡h1jL<br />

~<br />

j2i 2 ˆ 11 1 ‡ 12 2 ‡;<br />

214

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