Discrete mathematics is the study of mathematical structures that can be
considered "discrete" (in a way analogous to discrete variables, having a
bijection with the set of natural numbers) rather than "continuous"
(analogously to continuous functions). Objects studied in discrete mathematics
include integers, graphs, and statements in logic. By contrast, discrete
mathematics excludes topics in "continuous mathematics" such as real numbers,
calculus or Euclidean geometry. Discrete objects can often be enumerated by
integers; more formally, discrete mathematics has been characterized as the
branch of mathematics dealing with countable sets (finite sets or sets with
the same cardinality as the natural numbers).