REFERENCES
- Lipshitz , L. and Nadel , M. 1978 . The Additive Structure of Models of Arithmetic . Proc. Am. Math. Soc. , 68 : 331 – 336 .
- Tzouvaras , A. Expanding discretely ordered groups to rings. In preparation
- Shepherdson , J. C. 1964 . A nonstandard model for a free variable fragment of number theory . Acad. Pol. Sci. , 12 ( 2 ) : 79 – 86 .