Let $V$ be a (finite) $n$-dimensional real vector space. Suppose that $v^1, ..., v^n \in V$ satisfy:
- For all $i \leq n$, $v^i_i \geq 0$ and $v^i_{j \neq i} \leq 0$.
- For all $i \leq n$, $\sum_{j \leq n} v^j_i \geq 0$.
Claim: For all $i \leq n$, some non-negative [edited] linear combination of $v^1, ..., v^n$ has $i^{th}$ component $\sum_{j \leq n} v^j_i$ and all other components zero.
Seems basic but I'm not quite seeing the proof and can't find a reference. Can anyone help? Much appreciated.