Polymathematics: is mathematics a single science or a set of arts?
V.I.Arnold
All mathematics is divided into three parts: cryptography (paid for by CIA,
KGB and the like), hydrodynamics (supported by manufacturers of atomic
submarines) and celestial mechanics (financed by military and by other
institutions dealing with missiles, such as NASA.).
Cryptography has generated number theory, algebraic geometry over finite
fields, algebra \footnote{The creator of modern algebra, Vi\`ete, was the
cryptographer of King Henry~I\/V of France.}, combinatorics and computers.
Hydrodynamics procreated complex analysis, partial derivative
equations, Lie groups and algebra theory, cohomology theory and
scientific computing.
Celestial mechanics is the origin of dynamical systems, linear algebra,
topology, variational calculus and symplectic geometry.
The existence of mysterious relations between all these different domains
is the most striking and delightful feature of mathematics
(having no rational explanation).