Boundary Non-Crossings of Additive Wiener Fields

Let $W_i=\{W_i(t),t\in {\mathbb{R}}_{+}\},i=1,2$ be two Wiener processes and $W_3=\{W_3(\mathbf{t}),\mathbf{t}\in {\mathbb{R}}_{+}^2\}$ be a two-parameter Brownian sheet, all three processes being mutually independent. We derive upper and lower bounds for the boundary non-crossing probability $$P_f=P\{W_1(t_1)+W_2(t_2)+W_3(\mathbf{t})+h(\mathbf{t})\le u(\mathbf{t}),\mathbf{t}\in {\mathbb{R}}_{+}^2\},$$ where $h, u: $\mathbb{R}$+^2\rightarrow $\mathbb{R}$+$ are two measurable functions. We show further that for large trend functions $\gamma f>0$ asymptotically when $\gamma \to \mathrm{\infty }$ we have that $\ln P_{\gamma f}$ is the same as $\ln P_{\gamma \underset{‾}{f}}$ where $\underset{‾}{f}$ is the projection of $f$ on some closed convex set of the reproducing kernel Hilbert Space of $W$. It turns out that our approach is applicable also for the additive Brownian pillow.