The Kolmogorov-Arnold representation theorem
Every continuous real function on the closed unit n-cube is a superposition of continuous functions of one variable and addition, with the inner functions universal for the dimension. Three forms, each implying the next, named for who is credited with the statement. Lorentz-Sprecher form: there are positive constants lam_1, ..., lam_n and continuous strictly increasing psi_0, ..., psi_2n : R -> R, all depending only on n, such that every f continuous on [0,1]^n is f(x) = sum_{q=0}^{2n} g(sum_{p=1}^{n} lam_p psi_q(x_p)) on the cube for some continuous g : R -> R depending on f. Lorentz's…
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