Level Set

Given a real function $f(x_1, \cdots, x_n)$, its level set for $f(x_1, \cdots, x_n) =c$ are the corresponding arguments $x_1, \cdots, x_n$¹,

$$ L_c(f) = \{ (x_1, \cdots, x_n) \vert f(x_1, \cdots, x_n) =c \} $$