Flat (geometry)

In geometry, a flat or Euclidean subspace is a subset of a Euclidean space that is itself a Euclidean space (of lower dimension). The flats in two-dimensional space are points and lines, and the flats in three-dimensional space are points, lines and planes.

In a n-dimensional space, there are flats of every dimension from $0$ to $n-1$; flats of dimension $n-1$ are called hyperplanes.

Flats are the affine subspaces of Euclidean spaces, which means that they are similar to linear subspaces, except that they need not pass through the origin. Flats occur in linear algebra, as geometric realizations of solution sets of systems of linear equations.

A flat is a manifold and an algebraic variety, and is sometimes called a linear manifold or linear variety to distinguish it from other manifolds or varieties.

1 Descriptions

---

1.1 By equations

---

A flat can be described by a system of linear equations. For example, a line in two-dimensional space can be described by a single linear equation involving $x,y$:
$3x+5y=8.$

In three-dimensional space, a single linear equation involving $x,y,z$ defines a plane, while a pair of linear equations can be used to describe a line. In general, a linear equation in $n$ variables describes a hyperplane, and a system of linear equations describes the intersecation of those hyperplanes. Assuming the equations are consistent and linearly independent, a system of $k$ equations describes a flat of dimension $n-k$.

1.2 Parametric

---

A flat can also be described by a system of linear parametric equations. A line can be described by equations involving one parameter:
$x=2+3t,y=-1+tz=\frac{3}{2}-4t$
while the description of a plane would require two parameters:
$x=5+2t_1-3t_2,y=-4+t_1+2t_{2}z=5t_1-3t_2.$

In general, a parameterization of a flat of dimension $k$ would require parameters $t_1,...,t_k$

2 Operations and relations on flats

---

2.1 Intersecting, parallel, and skew flats

---

An intersection of flats is either a flat or the empty set.

If each line from one flat is parallel to some line from another flat, then these two flats are parallel. Two parallel flats of the same dimension either coincide or do not intersect; they can be described by two systems of linear equations which differ only in their right-hand sides.

If flats do not intersect, and no line from the first flat is parallel to a line from the second flat, then these are skew flats.

2.2 Join

---

For two flats of dimensions $k_1,k_2$ there exists the minimal flat which contains them, of dimension at most $k_1+k_2+1$. If two flats intersect, then the dimension of the containing flat equals to $k_1+k_2$ minus the dimension of the intersection.

2.3 Properties of operations

---

These two operations (referred to as meet and join) make the set of all flats in the Euclidean $n$-space a lattice and can build systematic coordinates for flats in any dimension, leading the Grassmann coordinates or dual Grassmann coordinates. For example, a line in three-dimensional space is determined by two distinct points or by two distince planes.

However, the lattice of all flats is not a distributive lattics. if two line $l_1,l_2$ intersect, then $l_1\cap l_2$ is a point. if $p$ is a point not lying on the same plane, then $(l_1\cap l_2)+p=(l_1+p)\cap (l_2+p)$, both representing a line. But when $l_1,l_2$ are parallel, this distributivity fails, giving $p$ on the left-hand side and a third parallel line on the right-hand side.

3 Euclidean geometry

---

The aforementioned facts do not depend on the structure being that of Euclidean space (namely, involving Euclidean distance) and are correct in any affine space. In a Euclidean space:

• There is the distance between a flat and a point. (See for example Distance from a point to a plane and Distance from a point to a line)
• There is the distance between two flats, equal to $0$ if they intersect. (See for example Distance between two lines (in the same plane) and Skew lines § Distance)
• There is the angle between two flats, which belongs to the interval $[0,\frac{\pi }{2}]$ between $0$ and the right angle. (See for example Dihedral angle (between two planes). See also Angles between flats).

4 See also

---

• N-dimensional space
• Matriod
• Coplanarity
• Isometry

5 Notes

@Skip

6 References

@Skip

7 External links

@Skip