who was a leading figure in introducing rigour to mathematical analysis via epsilon-delta notation. To show that intuition is not always reliable, he gave an example of a function that is continuous at every point but nowhere differentiable. He also proved the isoperimetric inequality and disproved Dirichlet’s principle. Weierstrass’ approach to complex analysis, via series, was in marked contrast to Riemann’s geometric approach, via Riemann surfaces.