  Building on the author's recent work with Jan Maas and Jan van Neerven, this
paper establishes the equivalence of two norms (one using a maximal function,
the other a square function) used to define a Hardy space on $\R^{n}$ with the
gaussian measure, that is adapted to the Ornstein-Uhlenbeck semigroup. In
contrast to the atomic Gaussian Hardy space introduced earlier by Mauceri and
Meda, the $h^{1}(\R^{n};d\gamma)$ space studied here is such that the Riesz
transforms are bounded from $h^{1}(\R^{n};d\gamma)$ to $L^{1}(\R^{n};d\gamma)$.
This gives a gaussian analogue of the seminal work of Fefferman and Stein in
the case of the Lebesgue measure and the usual Laplacian.

Functional Analysis