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can someone please give me a quick description/definition of a completeness relation in QM?

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- Thread starter nolanp2
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- #1

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can someone please give me a quick description/definition of a completeness relation in QM?

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malawi_glenn

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Here you can get some info:

http://phyastweb.la.asu.edu/phy576-schmidt/dirac/index.html [Broken]

http://phyastweb.la.asu.edu/phy576-schmidt/dirac/index.html [Broken]

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blechman

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[tex]

\{\phi_n(x)\} {\rm complete} \Rightarrow \psi(x)=\sum_n a_n\phi_n(x)

[/tex]

In particular, if you can find a set of energy eigenfunctions (which are complete due to theorems about hermitian operators on Hilbert spaces), you can always decompose any wavefunction in terms of them. This is very useful.

Slightly more formally, a complete set of functions satisfies:

[tex]

\sum_n \phi_n(x)\phi_n(y)=\delta(x-y)

[/tex]

The above follows (more or less) from these hypotheses.

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Quoting from J. J. Sakurai - Modern Quantum Mechanics, Sec. 1.3

... we must have

[tex]\sum_{a'} \left| a' \right\rangle \left\langle a' \right| = 1[/tex] (1.3.11)

where the 1 on the right-hand side is to be understood as the identityoperator. Equation (1.3.11) is known as thecompleteness relationorclosure.

The [tex]\left| a' \right\rangle[/tex] signify orthonormal eigenkets. The above sum is a useful mathematical tool as it can be inserted wherever the identity operator could appropriately be inserted. Check out the book!

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