Bolzano's Continuous but Nowhere Differentiable Function
Bolzano discovered this continuous but nowhere differentiable function before 1831, but these investigations were not published until 1930. Weierstrass found an analogous function in 1875. The function is the limit of the ones graphed as .
K. A. Rybnikov, A History of Mathematics (in Russian), Moscow: Moscow University Press, 1974 pp. 336–339.