David Hilbert was a German mathematician and one of the most
influential mathematicians of the 19th and early 20th centuries.
Hilbert discovered and developed a broad range of fundamental
ideas in many areas,
including invariant theory, the calculus of variations,
commutative algebra, algebraic number theory,
the foundations of geometry, spectral theory of operators and its
application to integral equations,
mathematical physics, and the foundations of mathematics
(particularly proof theory).