I have an algorithm with running time that satisfies $$ T(n) \leq n + \frac{1}{n}\sum^{n-1}_{i=0}(T(i) + T(n-i)),$$ and $T(0) = 0$. I was able to show that $T(n) = \mathcal O(n\log n)$ with a leading constant $c > 4$.
I modified this algorithm such that the recursion for $T(n)$ now satisfies $$ T(n) \leq 3 \log n + n + \frac{3}{n}\sum^{2n/3}_{i=n/3}(T(i) + T(n-i)),$$ and $T(0) = 0$. Intuitively, this algorithm should also be $\mathcal O(n\log n)$ but with a better leading constant, but I haven't been able to show it. Any ideas?