Past exam of the mathematics course of the University of Cambridge / 2013 / ia / Paper 4 / 6E / iii

For bounded weak compositions, start with the unrestricted stars and bars set $S$ and let $A_i$ be its subset where $n_i\ge a_i$. For an index subset $J\subseteq \{0,\dots ,r\}$, subtract $a_i$ from each coordinate with $i\in J$. This is a bijection from ${\bigcap }_{i\in J}{A}_i$ to nonnegative tuples summing toThuswhere a negative remaining sum gives zero, in accordance with the stipulated convention. Apply the avoidance form of the inclusion-exclusion principle:Expanding by subset size gives the alternating single-index, pair-index and higher-index sums. The empty subset supplies $\left(\genfrac{}{}{0ex}{}{N+r}{r}\right)$.