
doi: 10.1007/bf03322163
According to statements made by K. Weierstras, B. Riemann claimed already in 1861 that the function \({\sum\limits^\infty_{n=1}\ \ {\rm sin}\ n^2x\over n^2}\) is not differentiable in a dense set of points. A proof of this fact was however not published before 1916. In this article we study new sources which imply that both Weierstra\sB and L. Kronecker already knew proofs and which back up the Riemannian origin of the function. Furthermore, a proof is given that besides some elementary epsilontics only uses information on the growth of the theta series at the boundary of its domain of convergence. Remarkably, this statement on the theta series can be found in a fragment by Riemann.
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