two pions propagating in $s$-wave, which will show the $f_0(500)$ and $f_0(980)$ resonances. In addition we study the $D^0
\to K^0_s \pi^0 \eta$ amplitude, where the $a_0(980)$ resonance shows up, and relate it to the former one. However, we show that, by using basic symmetries and the chiral unitary approach to deal with the meson meson interaction in coupled channels, one is able to determine the shapes of the different amplitudes and the relative weight to each other with no free parameters. Hence genuine predictions for the shapes of these amplitudes and the relative weights of $f_0(500)$, $f_0(980)$ and $a_0(980)$ can be made and compared with experiment.

The chiral unitary approach for meson meson interaction makes use of the Bethe Salpeter (BS) equation in coupled channels. One takes all possible meson meson channels that couple within $SU(3)$ to certain given quantum numbers and the BS equation guaranties exact unitary. The kernel (potential) for the BS equation is taken from the chiral Lagrangians [@Gasser:1983yg; @Bernard:1995dp] and there is freedom for only some regularization scale in the meson meson loops, which is fitted to the meson meson scattering data. A good agreement with experimental data is obtained up to 1.2 GeV [@npa; 