While trying to analyse the runtime of an algorithm, I arrive to a recurrence of the following type :

$ $ \begin{cases} T(n) = \Theta(1), & \text{for small enough $ n$ ;}\ T(n) \leq T(a_n n + h(n)) + T((1-a_n)n+h(n)) + f(n), & \text{for larger $ n$ .} \end{cases}$ $ Where:

- $ a_n$ is unknown and can depend on $ n$ but is bounded by two constants $ 0<c_1\leq a_n \leq c_2 < 1$ .
- $ h(n)$ is some “fudge term” which is negligeable compared to $ n$ (say $ O(n^\epsilon)$ for $ 0\leq \epsilon < 1$ ).

If $ a_n$ was a constant, then I could use the Akra-Bazzi method to get a result. On the other hand, if the fudge term didn’t exist, then some type of recursion-tree analysis would be straight-forward.

To make things a bit more concrete, here is the recurrence I want to get the asymptotic growth of:

$ $ \begin{cases} T(n) = 1, & \text{for n = 1;}\ T(n) \leq T(a_n n + \sqrt n) + T((1-a_n)n+\sqrt n) + n, & \text{for $ n \geq 2$ } \end{cases}$ $ Where $ \frac{1}{4} \leq a_n \leq \frac{3}{4}$ for all $ n\geq 1$ .

I tried various guesses on upper bounds or transformations. Everything tells me this should work out to $ O(n\log(n))$ and I can argue informaly that it does (although I might be wrong). But I can only prove $ O(n^{1+\epsilon})$ (for any $ \epsilon>0$ ), and this feels like something that some generalization of the Master theorem à la Akra-Bazzi should be able to take care of.

Any suggestions on how to tackle this type of recurrence?