Riemann's innovative published works constructed the base for what is known as modern mathematics and research areas including analysis and geometry. These works finally proved to be very useful in the theories of algebraic geometry, Riemannian geometry and complex manifold theory. Adolf Hurwitz and Felix Klein comprehensively explained the theory of Riemann surfaces. This aspect of mathematics is the groundwork of topology, and is still extensively applied in modern mathematical physics. Riemann also established some breakthrough milestones in the theory of ‘Real Analysis’. He explained ‘the Riemann integral’ by means of Riemann sums and penned down a theory of trigonometric series that are not Fourier series, a first step in generalized function theory, and also explored the ‘Riemann–Liouville differintegral’.