Vectors
Contents
Vectors#
The term vector is used in several different contexts, each coming with a slightly different definition. In basic linear algebra a vector often is considered a finite column of numbers. That’s the approach we follow here.
Definition#
For \(d\in\mathbb{N}\) a \(d\)-tuple is an ordered list of real numbers, typically written as \((x_1,x_2,\ldots,x_d)\) with \(x_1,\ldots,x_d\) denoting the numbers. Here ‘ordered’ means that swapping two unequal numbers in the list yields a different \(d\)-tuple. Example: \((1,2,3)\neq(1,3,2)\). By \(\mathbb{R}^d\) we denote the set of \(d\)-tuples.
Vector is another term for \(d\)-tuple. In linear algebra vectors may be interpreted as points in space or as difference between two points (that is, describing a translation). Vectors often are written as columns:
Length of a Vector#
The (Euclidean) length of a vector \(x\) is defined as
Sum of Vectors#
Sums of vectors are defined componentwise:
Multiples of Vectors#
Products of real numbers and vectors are defined componentwise. For \(a\in\mathbb{R}\) and \(x\in\mathbb{R}^d\) we have
Inner Products#
The inner product of two vectors \(x,y\in\mathbb{R}^d\) is
Inner products are closely related to angles between vectors.
Outer Products#
The outer product of two vectors \(x,y\in\mathbb{R}^3\) is
The outer product yields a vector orthogonal to both factors.