EN
We extend the classical theorems of I. I. Privalov and A. Zygmund from single to multiple conjugate functions in terms of the multiplicative modulus of continuity. A remarkable corollary is that if a function f belongs to the multiplicative Lipschitz class $Lip(α₁,..., α_N)$ for some $0 < α₁,...,α_N < 1$ and its marginal functions satisfy $f(·,x₂,...,x_N) ∈ Lip β₁,...,f(x₁,...,x_{N-1},·) ∈ Lip β_N$ for some $0 < β₁,...,β_N < 1$ uniformly in the indicated variables $x_{l}$, 1 ≤ l ≤ N, then $f̃^{(η₁, ..., η_N)} ∈ Lip(α₁, ..., α_N)$ for each choice of $(η₁,...,η_N)$ with $η_{l} = 0$ or 1 for 1 ≤ l ≤ N.