Solution

ID: past-exam-of-the-model-theory-course-of-the-university-of-oxford/2011/1/b/ii/gamma/solution

No. Use the prime-sign automorphism of the multiplicative integers
where is the exponent of two in , a P-adic valuation. For nonzero integers, , so ; the identity also holds when a factor is zero. The exponent of two is unchanged by changing the sign, hence . Thus is a multiplicative structure automorphism.
It fixes one and sends two to minus two. The addition-graph point is sent to , outside the graph of addition. By automorphism invariance of parameter-free definability, addition is not definable in the multiplicative reduct.

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