Bellman functions and $L^p$ estimates for paraproducts

We give an explicit formula for one possible Bellman function associated with the $L^p$ boundedness of dyadic paraproducts regarded as bilinear operators or trilinear forms. Then we apply the same Bellman function in various other settings, to give self-contained alternative proofs of the estimates for several classical operators. These include the martingale paraproducts of Ba~{n}uelos and Bennett and the paraproducts with respect to the heat flows.