We show that the Hardy
space Hanal1(R+2× R+2) can be identified with the class of functions f such that f
and all its double and partial Hilbert transforms Hkf belong to L1(R2). A
basic tool used in the proof is the bisubharmonicity of |F|q, where F is a
vector field that satisfies a generalized conjugate system of Cauchy-Riemann
type.