Number theory is a branch of mathematics dealing with the properties of numbers, especially whole, as well as the broader class of problems that arise from this study. The term arithmetic is also used in number theory. This is an older term that is no longer as popular as it once was. Number theory was called the higher arithmetic, but this term is no longer in use. However, arithmetic expression persists in the names of some mathematical fields (arithmetic functions, arithmetic of elliptic curves, fundamental theorem of arithmetic). This sense of the term arithmetic should not be confused with elementary arithmetic is not, nor the branch of logic that studies Peano arithmetic as a formal system. Mathematicians who deal with the theory of numbers are called number theorists.