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

M. Knebusch, A. Rosenberg, and R. Ware, `Structure of Witt rings and quotients of abelian 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.