## Quantum Theory: Concepts and Methods (Dordrecht

- A Peres
- Kluwer) p
- 1993

- Published 1998

The momentum operator for a particle in a box is represented by an infinite order Hermitian matrix P . Its square P 2 is well defined (and diagonal), but its cube P 3 is ill defined, because P P 2 6= P 2 P . Truncating these matrices to a finite order restores the associative law, but leads to other curious results. Classification 0260 (02.10.Sp)

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