A **vector** is an ordered finite list of numbers. Its entries are called **elements** of the vector. The **dimension** of the vector is the number of elements it contains.

Denoting an \(n\)-dimensional vector using the symbol \(\pmb{a}\), the \(i\)-th element of the vector \(\pmb{a}\) is denoted with \(a_i\), where the subscript \(i\) is an integer index that runs from 1 to \(n\).

A vector is said to be **sparse** if many of its elements are zero, i.e. if \(a_i=0\) for many \(i\).