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Is the product of two Cesaro convergent series Cesaro convergent?

Mathematics Asked by Antonio Claire on November 9, 2021

Let ${a_n }_{n geq 1}$ and ${b_n }_{n geq 1}$ be two sequences of real numbers such that the infinite series $sumlimits_{n=1}^{infty} a_n$ and $sumlimits_{n=1}^{infty} b_n$ are both convergent in the Cesaro sense i.e. begin{align*} limlimits_{n to infty} frac 1 n sumlimits_{k=1}^{n} s_k & < + infty \ limlimits_{n to infty} frac 1 n sumlimits_{k=1}^{n} t_k & < + infty end{align*}

where ${s_k }_{k geq 1}$ and ${t_k}_{k geq 1}$ are sequences of partial sums of the series $sumlimits_{n=1}^{infty} a_n$ and $sumlimits_{n=1}^{infty} b_n$ respectively. Can I say that $sumlimits_{n=1}^{infty} a_n b_n$ is convergent in the Cesaro sense? If "yes" then what can I say about it’s limit in terms of the limits of the given two series?

One Answer

No. Consider $a_{n}=b_n=(-1)^n$. Then both of them are Cesaro summable but $c_n=a_n cdot b_n= 1$ isn't, since $limlimits_{n to infty} frac 1 n sumlimits_{k=1}^{n} u_k= limlimits_{nto infty}frac{1}{n}frac{(n+1)n}{2}=infty$

Answered by alphaomega on November 9, 2021

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