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It is known that $$\int_T^{2T}|\zeta^{(m)}(\tfrac{1}{2}+it)|^{2k}dt\gg T(\log T)^{k^2+2km}.$$ I want to establish a lower bound for $$\int_T^{2T}|\zeta^{(m)}(\tfrac{1}{2}+it)|^{2k}|\zeta^{(n)}(\tfrac{1}{2}+it)|^{2\ell}dt,$$ but cannot see any way of doing this. For simplicity we can assume $k,\ell$ to be positive integers.

My first plan was to sub in the approximate functional equation for both, but it doesn't seem as though this will give any result at all, especially due to the interlinking of different derivatives $m$ and $n$. Is there any way to modify the proofs of the first equation to establish any bounds on the latter?

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This is proved for real non-negative $k$ and $\ell$ in Chapter 2 of my thesis (I'm currently making minor corrections to it, and a version of this Chapter should appear on the arXiv before too long).

One adapts the lower bounds method of Radziwi{\l}{\l}-Soundararajan (https://arxiv.org/abs/1202.1351v1), or more relevantly for the Riemann zeta function Heap-Soundararajan's work (https://arxiv.org/abs/2007.13154).

Very briefly, with a generalisation of the Hölder's inequality in Heap--Soundararajan's paper one has (assuming that $k\geq \ell$): \begin{equation}\label{eq:cts-lb-holder-1} \begin{split} \Big|\int_T^{2T}\zeta^{(m)}(\tfrac{1}{2}+it)&\zeta^{(n)}(\tfrac{1}{2}+it)|\mathcal{N}(\tfrac{1}{2}+it)|^2 \ d t\Big|\\ \ll & \Big(\int_T^{2T}|\zeta^{(m)}(\tfrac{1}{2}+it)|^{2k}|\zeta^{(n)}(\tfrac{1}{2}+it)|^{2\ell}\ d t\Big)^{1/(2u(k+\ell))}\\ &\quad \times\Big(\int_T^{2T}|\mathcal{N}(\tfrac{1}{2}+it)|^{2+2/(k+\ell-1)}\ d t\Big)^{(k+\ell-1)/(2u(k+\ell))}\\ &\quad \times{\Big(\int_T^{2T}|\zeta^{(m)}(\tfrac{1}{2}+it)|^2|\mathcal{N}(\tfrac{1}{2}+it)|^2\ d t\Big).}^{1-1/(2u)} \end{split} \end{equation} where $u=\ell/(k+\ell)$ and the notation $\mathcal{N}(s,\cdot)$ is the same as in Heap-Soundararajan. The idea is that the $\mathcal{N}$ is a Dirichlet polynomial approximation to zeta that ensures that Hölder's inequality is sharp-up-to-constant and each factor has the same order of magnitude.

The fractional Dirichlet polynomial moment is essentially the same as Heap-Soundararajan's and the twisted moment calculations can be done by differentiating the usual formulae for the twisted second and fourth moments of zeta.

By the way, is there a particular application that you were thinking of for this result?

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