Let $$f_{a,b}(x)=x^3+(u-1-a-b)x^2+ax+b.$$ where $$ f_{a,b}(x)=x^3+(u-1-a-b)x^2+ax+b, $$ where $$u\in\mathbb{Z}_p^*$$$u\in\mathbb{Z}_p^*$ is fixed . Let Let $$S$$$S$ be the set consisting of all pairs $$(a,b)\in\mathbb{Z}_p^2$$$(a,b)\in\mathbb{Z}_p^2$ such that $$f_{a,b}(x)$$$f_{a,b}(x)$ factor linearly.Then what Then what is the cardinality of S$S$? Is it possible to get an exact formula somehow ?
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