Per the title, what are some of the oldest books on series out there with unsolved exercises? Maybe there are some hidden gems from before the 20th century out there.



Reference request: Oldest books on series with unsolved exercises?
Two classics from a century ago, with a great variety of unsolved exercises:


[Theorie und Anwendung der unendlichen Reihen](https://archive.org/details/theorieundanwen00knopgoog/page/n14), by [Konrad Knopp](https://en.wikipedia.org/wiki/Konrad_Knopp) (1922) --- [English translation](https://www.maa.org/press/maa-reviews/theory-and-application-of-infinite-series)


[An Introduction to the Theory of Infinite Series](https://archive.org/details/introductiontoth00bromuoft/page/n6) by [Thomas Bromwich](https://en.wikipedia.org/wiki/Thomas_John_I%27Anson_Bromwich) (1908)


An older but more elementary text is [Introduction to Infinite Series](https://archive.org/details/introinfiniteser00osgorich) by [William Osgood](https://en.wikipedia.org/wiki/William_Fogg_Osgood) (1897)


An exercise from Knopp:
![](https://ilorentz.org/beenakker/MO/Knopp.png)


An exercise from Bromwich:
![](https://ilorentz.org/beenakker/MO/Bromwich.png)


An exercise from Osgood:
![](https://ilorentz.org/beenakker/MO/Osgood.png)


