\prod{\left( {t - s,\omega - \theta} \right)W_{y_{a}}\left( {s,\theta} \right)\text{d}s\text{d}\theta}}}}{\int_{- \infty}^{\infty}{\int_{- \infty}^{\infty}{\prod{\left( {t - s,\omega - \theta} \right)W_{y_{a}}\left( {s,\theta} \right)\text{d}s\text{d}\theta}}}}$$