Let $W=\{w_k: 1\le k\le N\}$ be sequence of nonzero, distinct, real numbers with $\sum\limits_{k\ge 1}\frac{1}{|w_k|}<\infty$ and $\xi_0$ be a fixed number in $(0,1)$. Find the uniform bound of $$f_N=\prod_{k=1}^{N}(1+e^{2\pi w_k})\sum_{k=1}^N a_{k,N}\frac{e^{2\pi \xi_0 w_k}}{1+e^{2\pi w_k}},$$ where $a_{k,N}=w_k\prod\limits_{j=1, j\ne k}\frac{w_j}{w_j-w_k}$. Can we find a finite positive number $C$ such that $$|f_N|\ge C>0,\text{ uniformly in }N.$$ I have tried it as follows. $f_N=P_NS_N$, where $P_N=\prod_{k=1}^{N}(1+e^{2\pi w_k})$ and $S_N=\sum_{k=1}^N a_{k,N}\frac{e^{2\pi \xi_0 w_k}}{1+e^{2\pi w_k}}$. Here $$S_N=\sum_{k=1}^{N} h(w_k)L_k(0),$$ where $L_k(x)=\prod\limits_{j=1, j\ne k}\frac{x-w_j}{w_k-w_j}$ is Lagrange basis function and $h(w)=w\frac{e^{2\pi \xi_0 w}}{1+e^{2\pi w}}$. If $w_k$ tends to $\infty$, then $Q_N$ blows up and $S_N$ decays rapidly, their product $|f_N|>C_1>0$ (verified numerically for large N using MATLAB). So maybe we cannot go separately. Moreover, I got $$\frac{1}{2\pi i}\int_{C_N} H_N(z)dz=-S_N+\psi(0).$$ where $\psi(z)=\frac{e^{2\pi \xi_0 z}}{1+e^{2\pi z}}$, $H_N(z)=\frac{\psi(z)}{zG_N(z)}$, $G_N(z)=\prod\limits_{j=1}^N(1-\frac{z}{w_j})$, and $C_N$ is a rectangle contour with $|y|<\frac{1}{2}$ contains poles $w_1,\cdots, w_N$. Since $\psi(z)$ has pole has at $z=(m+\frac{1}{2})i,~m\in\mathbb{Z}$, $C_N$ contains no poles on imaginary axis.
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1 - $\begingroup$ When you ask a question on this site, make sure to also show what you have tried. It is essential to demonstrate that you have spent some time on this problem. Simply putting the question will lead to downvotes and the closing of the post. $\endgroup$Afntu– Afntu2025-11-08 12:59:17 +00:00Commented Nov 8 at 12:59
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