# Centroid

of a set of points in Euclidean space

Consider $m$ points $a _ {i}$, $i = 1 \dots m$, in $\mathbf R ^ {n}$, with positive weights $t _ {i}$ attached to them. Then the centroid of this system is the point

$$c = { \frac{\sum _ {i = 1 } ^ { m } t _ {i} a _ {i} }{\sum _ {i = j } ^ { m } t _ {i} } } .$$

This is also the centre of gravity, or barycentre, or centre of mass (in the sense of mechanics) of these mass points. The concept belongs to affine geometry rather than to the vector space (Euclidean space) $\mathbf R ^ {n}$, in the sense that the vector

$$x + \left ( \sum _ {i = 1 } ^ { m } t _ {i} \right ) ^ {- 1 } \sum _ {i = 1 } ^ { m } t _ {i} ( a _ {i} - x )$$

is obviously independent of $x$.

If $t _ {i} = 1$, $i = 1 \dots m$, the term equi-barycentre is sometimes used. The concept can be extended to the case of possibly negative $t _ {i}$( electric charges), see [a1].

A related concept is that of the centre of a compact set $K$ with non-empty interior in $\mathbf R ^ {n}$, which is defined as:

$$c = \mu ( K ) ^ {- 1 } \int\limits _ { K } x {d \mu } ,$$

where $\mu$ is Lebesgue measure. This is the centre of mass, in the sense of mechanics, of the body $K$ with uniform mass density. Here, a weight function (mass distribution) could be introduced.

For a triangle, the centroid of the three corners, each with weight unity, is the intersection point of the three medians of the triangle (see Plane trigonometry; Median (of a triangle)). This is also the centre of the convex hull of the three corners (Archimedes' theorem). This is no longer necessarily true for four points in the plane.

The centroid of $a _ {1} \dots a _ {m}$ with weights $t _ {i}$, $i = 1 \dots m$, minimizes the weighted sum of the squared distances $\sum _ {i = 1 } ^ {m} t _ {i} \| {x - a _ {i} } \| ^ {2}$.