The diagonals of a cube with side length 1. AC' (shown in blue) is a space diagonal with length ${\displaystyle {\sqrt {3}}}$, while AC (shown in red) is a face diagonal and has length ${\displaystyle {\sqrt {2}}}$.

In geometry, a diagonal is a line segment joining two vertices of a polygon or polyhedron, when those vertices are not on the same edge. Informally, any sloping line is called diagonal. The word diagonal derives from the ancient Greek διαγώνιος diagonios,^[1] "from angle to angle" (from διά- dia-, "through", "across" and γωνία gonia, "angle", related to gony "knee"); it was used by both Strabo^[2] and Euclid^[3] to refer to a line connecting two vertices of a rhombus or cuboid,^[4] and later adopted into Latin as diagonus ("slanting line").

In matrix algebra, the diagonal of a square matrix consists of the entries on the line from the top left corner to the bottom right corner.

There are also many other non-mathematical uses.

Non-mathematical uses

A stand of basic scaffolding on a house construction site, with diagonal braces to maintain its structure

In engineering, a diagonal brace is a beam used to brace a rectangular structure (such as scaffolding) to withstand strong forces pushing into it; although called a diagonal, due to practical considerations diagonal braces are often not connected to the corners of the rectangle.

Diagonal pliers are wire-cutting pliers defined by the cutting edges of the jaws intersects the joint rivet at an angle or "on a diagonal", hence the name.

A diagonal lashing is a type of lashing used to bind spars or poles together applied so that the lashings cross over the poles at an angle.

In association football, the diagonal system of control is the method referees and assistant referees use to position themselves in one of the four quadrants of the pitch.

Polygons

As applied to a polygon, a diagonal is a line segment joining any two non-consecutive vertices. Therefore, a quadrilateral has two diagonals, joining opposite pairs of vertices. For any convex polygon, all the diagonals are inside the polygon, but for re-entrant polygons, some diagonals are outside of the polygon.

Any n-sided polygon (n ≥ 3), convex or concave, has ${\displaystyle {\tfrac {n(n-3)}{2}}}$ total diagonals, as each vertex has diagonals to all other vertices except itself and the two adjacent vertices, or n − 3 diagonals, and each diagonal is shared by two vertices.

In general, a regular n-sided polygon has ${\displaystyle \lfloor {\frac {n-2}{2}}\rfloor }$ distinct diagonals in length, which follows the pattern 1,1,2,2,3,3... starting from a square.

Polyhedrons

A polyhedron (a solid object in three-dimensional space, bounded by two-dimensional faces) may have two different types of diagonals: face diagonals on the various faces, connecting non-adjacent vertices on the same face; and space diagonals, entirely in the interior of the polyhedron (except for the endpoints on the vertices).

Higher dimensions

N-Cube

The lengths of an n-dimensional hypercube's diagonals can be calculated by mathematical induction. The longest diagonal of an n-cube is ${\displaystyle {\sqrt {n}}}$. Additionally, there are ${\displaystyle 2^{n-1}{\binom {n}{x+1}}}$ of the xth shortest diagonal. As an example, a 5-cube would have the diagonals:

Diagonal length Number of diagonals ${\displaystyle {\sqrt {2}}}$ 160 ${\displaystyle {\sqrt {3}}}$ 160 2 80 ${\displaystyle {\sqrt {5}}}$ 16

Its total number of diagonals is 416. In general, an n-cube has a total of ${\displaystyle 2^{n-1}(2^{n}-n-1)}$ diagonals. This follows from the more general form of ${\displaystyle {\frac {v(v-1)}{2}}-e}$ which describes the total number of face and space diagonals in convex polytopes.^[9] Here, v represents the number of vertices and e represents the number of edges.

Matrices

For a square matrix, the diagonal (or main diagonal or principal diagonal) is the diagonal line of entries running from the top-left corner to the bottom-right corner.^[10][11][12] For a matrix ${\displaystyle A}$ with row index specified by ${\displaystyle i}$ and column index specified by ${\displaystyle j}$, these would be entries ${\displaystyle A_{ij}}$ with ${\displaystyle i=j}$. For example, the identity matrix can be defined as having entries of 1 on the main diagonal and zeroes elsewhere:

${\displaystyle {\begin{pmatrix}1&0&0\\0&1&0\\0&0&1\end{pmatrix}}}$

The trace of a matrix is the sum of the diagonal elements.

The top-right to bottom-left diagonal is sometimes described as the minor diagonal or antidiagonal.

The off-diagonal entries are those not on the main diagonal. A diagonal matrix is one whose off-diagonal entries are all zero.^[13][14]

A superdiagonal entry is one that is directly above and to the right of the main diagonal.^[15][16] Just as diagonal entries are those ${\displaystyle A_{ij}}$ with ${\displaystyle j=i}$, the superdiagonal entries are those with ${\displaystyle j=i+1}$. For example, the non-zero entries of the following matrix all lie in the superdiagonal:

${\displaystyle {\begin{pmatrix}0&2&0\\0&0&3\\0&0&0\end{pmatrix}}}$

Likewise, a subdiagonal entry is one that is directly below and to the left of the main diagonal, that is, an entry ${\displaystyle A_{ij}}$ with ${\displaystyle j=i-1}$.^[17] General matrix diagonals can be specified by an index ${\displaystyle k}$ measured relative to the main diagonal: the main diagonal has ${\displaystyle k=0}$; the superdiagonal has ${\displaystyle k=1}$; the subdiagonal has ${\displaystyle k=-1}$; and in general, the ${\displaystyle k}$-diagonal consists of the entries ${\displaystyle A_{ij}}$ with ${\displaystyle j=i+k}$.

A banded matrix is one for which its non-zero elements are restricted to a diagonal band. A tridiagonal matrix has only the main diagonal, superdiagonal, and subdiagonal entries as non-zero.

Geometry

By analogy, the subset of the Cartesian product X×X of any set X with itself, consisting of all pairs (x,x), is called the diagonal, and is the graph of the equality relation on X or equivalently the graph of the identity function from X to X. This plays an important part in geometry; for example, the fixed points of a mapping F from X to itself may be obtained by intersecting the graph of F with the diagonal.

In geometric studies, the idea of intersecting the diagonal with itself is common, not directly, but by perturbing it within an equivalence class. This is related at a deep level with the Euler characteristic and the zeros of vector fields. For example, the circle S¹ has Betti numbers 1, 1, 0, 0, 0, and therefore Euler characteristic 0. A geometric way of expressing this is to look at the diagonal on the two-torus S¹xS¹ and observe that it can move off itself by the small motion (θ, θ) to (θ, θ + ε). In general, the intersection number of the graph of a function with the diagonal may be computed using homology via the Lefschetz fixed-point theorem; the self-intersection of the diagonal is the special case of the identity function.

See also

• Jordan normal form
• Main diagonal
• Diagonal functor

Notes

References

• Bronson, Richard (1970), Matrix Methods: An Introduction, New York: Academic Press, LCCN 70097490
• Cullen, Charles G. (1966), Matrices and Linear Transformations, Reading: Addison-Wesley, LCCN 66021267
• Herstein, I. N. (1964), Topics In Algebra, Waltham: Blaisdell Publishing Company, ISBN 978-1114541016
• Nering, Evar D. (1970), Linear Algebra and Matrix Theory (2nd ed.), New York: Wiley, LCCN 76091646

External links

Look up diagonal in Wiktionary, the free dictionary.

• Diagonals of a polygon with interactive animation
• Polygon diagonal from MathWorld.
• Diagonal of a matrix from MathWorld.