R. Ware,
`Valuation rings and rigid elements in fields,'
*Canadian Journal of Mathematics,* 33, 1981, 1338-1355.

M. Knebusch, A. Rosenberg, and R. Ware,
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group rings,'

* American Journal of Mathematics, * 94, 1972, 119-155.

R. Ware,
`Endomorphism rings of projective modules,'
* Transactions of the American Mathematical Society,*
155, 1971, 233-256.