Because the gradient vectors here are f sub x, z If you have a third variable z gz then you have also an equation f sub z equals lambda g sub z.
因为这里，梯度向量就是，如果还有一个变量,那就还有一个等式fz等于λ乘以。
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In particular, some upper bounds of the vertex arboricity of the integer distance graph G(Z, D) are obtained when D is a set of positive integers and Z the set of all integers.