= Essential variable of a function
= Essential variable
{synonym}
A coordinate of a <function> on a finite <Cartesian product> is essential if changing that coordinate, with all the others fixed suitably, can change the output. A coordinate is inessential precisely when such a change never affects the output in any context. If only coordinate $i$ is essential, changing the other coordinates one at a time proves that the output is $g(x_i)$ for a nonconstant unary <function> $g$. This deterministic dependence definition is not by itself a probabilistic or experimental notion of <causal inference>.
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