Necessary conditions for the positivity of Littlewood-Richardson and plethystic coefficients

We give necessary conditions for the positivity of Littlewood-Richardson coefficients and SXP coefficients. We deduce necessary conditions for the positivity of the general plethystic coefficients. Explicitly, our Main Result states that if $S^\lambda(V)$ appears as a summand in the decomposition into irreducibles of $S^\mu(S^\nu(V))$ then $\nu$'s diagram is contained in $\lambda$'s diagram.