On the resonant and non-resonant contributions to B\to\rho\pi

We discuss the importance of the background in order to understand the scattering in the J^{PC}=0^{++} low and intermediate energy region and in particular regarding the \sigma meson. In order to appreciate better its importance we compare with the \rho meson in the P-wave \pi\pi scattering. We also point out that in present analyses of three-body heavy meson decays, like those of D^+ and B, the role of this background is still not properly settled although it happens to be considerably smaller.