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Revision difference : non-existence of universal series convergence criterion |
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Version 7 |
| There exist many criteria for examining the convergence and \PMlinkescapetext{divergence} of series with positive \PMlinkescapetext{terms} (see e.g. determining series convergence).\, They all are sufficient but not necessary.\, It has also been asked whether there would be any criterion which were both sufficient and necessary.\, The famous mathematician Niels Henrik Abel took this question under consideration and proved the |
There exist many criteria for examining the convergence and \PMlinkescapetext{divergence} of series with positive \PMlinkescapetext{terms} (see e.g. determining series convergence).\, They all are sufficient but not necessary.\, It has also been asked whether there would be any criterion which were both sufficient and necessary.\, The famous mathematician Niels Henrik Abel took this question under consideration and proved the |
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| \begin{thmplain} |
\begin{thmplain} |
| There is no sequence |
There is no sequence |
| \begin{align} |
\begin{align} |
| \varrho_1,\,\varrho_2,\,\varrho_3,\,\ldots |
\varrho_1,\,\varrho_2,\,\varrho_3,\,\ldots |
| \end{align} |
\end{align} |
| of positive numbers such that every series\, $a_1\!+\!a_2\!+\!a_3\!+\cdots$\, of positive \PMlinkescapetext{terms} converges when the condition |
of positive numbers such that every series\, $a_1\!+\!a_2\!+\!a_3\!+\cdots$\, of positive \PMlinkescapetext{terms} converges when the condition |
| $$\lim_{n\to\infty}\varrho_n a_n = 0$$ |
$$\lim_{n\to\infty}\varrho_n a_n = 0$$ |
| is true but diverges when it is false. |
is true but diverges when it is false. |
| \end{thmplain} |
\end{thmplain} |
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| {\em Proof.}\, Let's assume that there is a sequence (1) having the both properties.\, We infer that the series\, $\frac{1}{\varrho_1}\!+\!\frac{1}{\varrho_2}\!+\!\frac{1}{\varrho_3}\!+\cdots$\, is divergent because\, $\lim_{n\to\infty}(\varrho_n\!\cdot\!\frac{1}{\varrho_n}) \ne 0$.\, The theorem on slower divergent series guarantees us another divergent series $s_1\!+\!s_2\!+\!s_3\!+\cdots$ such that the ratio \, $s_n\colon\!\frac{1}{\varrho_n} = |
{\em Proof.}\, Let's assume for a while that there is a sequence (1) having the both properties.\, We infer that the series\, $\frac{1}{\varrho_1}\!+\!\frac{1}{\varrho_2}\!+\!\frac{1}{\varrho_3}\!+\cdots$\, is divergent because\, $\lim_{n\to\infty}(\varrho_n\!\cdot\!\frac{1}{\varrho_n}) \ne 0$.\, The theorem on slower divergent series guarantees us another divergent series $s_1\!+\!s_2\!+\!s_3\!+\cdots$ such that the ratio \, $s_n\colon\!\frac{1}{\varrho_n} = |
| \varrho_n s_n$\, tends to the limit $0$ as\, $n\to\infty$.\, But this limit result concerning the series\, $s_1\!+\!s_2\!+\!s_3\!+\cdots$\, should \PMlinkescapetext{mean}, according to our assumption, that the series is convergent.\, The contradiction shows that the theorem holds. |
\varrho_n s_n$\, tends to the limit $0$ as\, $n\to\infty$.\, But this limit result concerning the series $s_1\!+\!s_2\!+\!s_3\!+\cdots$ should \PMlinkescapetext{mean}, according to our assumption, that the series is convergent.\, The contradiction shows that the theorem holds. |
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| \begin{thebibliography}{9} |
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| \bibitem{EL}{\sc Ernst Lindel\"of:} {\em Differentiali- ja integralilasku ja sen sovellutukset} III.2.\, Mercatorin Kirjapaino Osakeyhti\"o. Helsinki (1940). |
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| \end{thebibliography} |
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