Diagonal
In geometry, a diagonal is a line segment joining two nonconsecutive vertices of a polygon or polyhedron. Informally, any sloping line is called diagonal. The word "diagonal" derives from the ancient Greek διαγώνιος diagonios,^{} "from angle to angle" (from διά dia, "through", "across" and γωνία gonia, "angle", related to gony "knee"); it was used by both Strabo^{} and Euclid^{} to refer to a line connecting two vertices of a rhombus or cuboid,^{} and later adopted into Latin as diagonus ("slanting line").
In matrix algebra, a diagonal of a square matrix is a set of entries extending from one corner to the farthest corner.
There are also other, nonmathematical uses.
Nonmathematical uses
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 wirecutting 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 nonconsecutive 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 reentrant polygons, some diagonals are outside of the polygon.
Any nsided polygon (n ≥ 3), convex or concave, has
or
diagonals, as each vertex has diagonals to all other vertices except itself and the two adjacent vertices, or n − 3 diagonals.





Matrices
In the case of a square matrix, the main or principal diagonal is the diagonal line of entries running from the topleft corner to the bottomright corner.^{}^{}^{} For a matrix with row index specified by and column index specified by , these would be entries with . For example, the identity matrix can be defined as having entries of 1 on the main diagonal and zeroes elsewhere:
The topright to bottomleft diagonal is sometimes described as the minor diagonal or antidiagonal. The offdiagonal entries are those not on the main diagonal. A diagonal matrix is one whose offdiagonal entries are all zero.^{}^{}
A superdiagonal entry is one that is directly above and to the right of the main diagonal.^{}^{} Just as diagonal entries are those with , the superdiagonal entries are those with . For example, the nonzero entries of the following matrix all lie in the superdiagonal:
Likewise, a subdiagonal entry is one that is directly below and to the left of the main diagonal, that is, an entry with .^{} General matrix diagonals can be specified by an index measured relative to the main diagonal: the main diagonal has ; the superdiagonal has ; the subdiagonal has ; and in general, the diagonal consists of the entries with .
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 S^{1} 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 twotorus S^{1}xS^{1} 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 selfintersection of the diagonal is the special case of the identity function.
See also
Notes
References
 Bronson, Richard (1970), Matrix Methods: An Introduction, New York: Academic Press, LCCN 70097490
 Cullen, Charles G. (1966), Matrices and Linear Transformations, Reading: AddisonWesley, LCCN 66021267
 Herstein, I. N. (1964), Topics In Algebra, Waltham: , ISBN 9781114541016
 Nering, Evar D. (1970), Linear Algebra and Matrix Theory (2nd ed.), New York: Wiley, LCCN 76091646