Bound on function of sum of powers

Question

Let $(x_1, \ldots, x_k)\in R^k$ and $n=n_1+\ldots +n_k$, with $n\in N_0$ and $0\leq n_i\leq n$ Consider function $M_n=\sum_{i=1, \textit{number of terms is $2\ell+1$}}^kx_i^{n_i}$-function with odd number of terms.

Find the lower bound of $M_n$.