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The momentum operator for a particle in a box is represented by an infiniteorder Hermitian matrix $P$. Its square $P^2$ is well defined (and diagonal),but its cube $P^3$ is ill defined, because $P P^2\neq P^2 P$. Truncating thesematrices to a finite order restores the associative law, but leads to othercurious results.Comment: final version in J. Phys. A28 (1995) 1765-177

Infinite matrices may violate the associative law