Pi is a number.
Decimal representation . A decimal representation of a non-negative real number r is an expression in the form of a series , traditionally written as a sum A mathematical constant is a special number , usually a real number , that is `` significantly interesting in some way '' . Number theory or , in older usage , arithmetic is a branch of pure mathematics devoted primarily to the study of the integers . The number is a mathematical constant , the ratio of a circle 's circumference to its diameter , commonly approximated as 3.14159 .