The number 1 next to the Nabla symbol (∇) indicates that the gradient is being taken with respect to the first vector variable, denoted as ##\nabla_1##. In the context of two vector variables, ##\mathbf{r_1} = (x_1, y_1, z_1)## and ##\mathbf{r_2} = (x_2, y_2, z_2)##, this notation clarifies which variable the gradient operation is applied to. Similarly, ##\nabla_2## would represent the gradient with respect to the second vector variable. Understanding this distinction is crucial for correctly interpreting vector calculus operations.
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