. Definition (theory): A theory T is a pair consisting of a language L for FOL and a set of formulasof L. L is called the language of T and is denoted by L(T). is called the set of nonlogical arioms of T and is denoted by NIAx(T) . The language of theory called Robinson arithmetic, (denoted by Q) is the language of arithmetic. The nonlogical axioms of Q are: . Problem: Show that r+ y y+x is not valid in Robinson's nonstandard model . Definition (theory): A theory T is a pair consisting of a language L for FOL and a set of formulasof L. L is called the language of T and is denoted by L(T). is called the set of nonlogical arioms of T and is denoted by NIAx(T) . The language of theory called Robinson arithmetic, (denoted by Q) is the language of arithmetic. The nonlogical axioms of Q are: . Problem: Show that r+ y y+x is not valid in Robinson's nonstandard model