Polar sine

Template:Short description In geometry, the polar sine generalizes the sine function of angle to the vertex angle of a polytope. It is denoted by psin.

Definition

Template:Mvar vectors in Template:Mvar-dimensional space

The interpretations of 3D volumes for left: a parallelepiped (Ω in polar sine definition) and right: a cuboid (Π in definition). The interpretation is similar in higher dimensions.

Let Template:Math (Template:Math) be non-zero Euclidean vectors in [[n-dimensional space|Template:Mvar-dimensional space]] (Template:Math) that are directed from a vertex of a parallelotope, forming the edges of the parallelotope. The polar sine of the vertex angle is:

$\mathrm{psin}({𝐯}_1,\dots ,{𝐯}_n)=\frac{\mathrm{\Omega }}{\mathrm{\Pi }},$

where the numerator is the determinant

$\begin{array}{cc}\mathrm{\Omega }& =det\left[\begin{array}{cccc}{𝐯}_1& {𝐯}_2& \cdots & {𝐯}_n\end{array}\right]=\left|\begin{array}{cccc}{v}_{11}& v_{21}& \cdots & v_{n1}\\ v_{12}& v_{22}& \cdots & v_{n2}\\ ⋮& ⋮& \ddots & ⋮\\ v_{1n}& v_{2n}& \cdots & v_{nn}\end{array}\right|\end{array},$

which equals the signed hypervolume of the parallelotope with vector edges

$\begin{array}{cc}{𝐯}_1& =(v_{11},v_{12},\dots ,v_{1n}{)}^T\\ {𝐯}_2& =(v_{21},v_{22},\dots ,v_{2n}{)}^T\\ & ⋮\\ {𝐯}_n& =(v_{n1},v_{n2},\dots ,v_{nn}{)}^T,\end{array}$

and where the denominator is the Template:Mvar-fold product

$\mathrm{\Pi }=\prod _{i=1}^n\Vert {𝐯}_i\Vert$

of the magnitudes of the vectors, which equals the hypervolume of the Template:Mvar-dimensional hyperrectangle with edges equal to the magnitudes of the vectors Template:Math, Template:Math, ... Template:Math rather than the vectors themselves. Also see Ericksson.

The parallelotope is like a "squashed hyperrectangle", so it has less hypervolume than the hyperrectangle, meaning (see image for the 3d case):

$|\mathrm{\Omega }|\le \mathrm{\Pi }⟹\frac{|\mathrm{\Omega }|}{\mathrm{\Pi }}\le 1⟹-1\le \mathrm{psin}({𝐯}_1,\dots ,{𝐯}_n)\le 1,$

as for the ordinary sine, with either bound being reached only in the case that all vectors are mutually orthogonal.

In the case Template:Math, the polar sine is the ordinary sine of the angle that is swept out if the first vector is rotated counterclockwise to the position of the second vector.

In higher dimensions

A non-negative version of the polar sine that works in any Template:Mvar-dimensional space can be defined using the Gram determinant. It is a ratio where the denominator is as described above. The numerator is

$|\mathrm{\Omega }|=\sqrt{det\left({\left[\begin{array}{cccc}{𝐯}_1& {𝐯}_2& \cdots & {𝐯}_n\end{array}\right]}^T\left[\begin{array}{cccc}{𝐯}_1& {𝐯}_2& \cdots & {𝐯}_n\end{array}\right]\right)},$

where the superscript T indicates matrix transposition. This can be nonzero only if Template:Math. In the case Template:Math, this is equivalent to the absolute value of the definition given previously. In the degenerate case Template:Math, the determinant will be of a singular Template:Math matrix, giving Template:Math and Template:Math, because it is not possible to have Template:Mvar linearly independent vectors in Template:Mvar-dimensional space when Template:Math.

Properties

Interchange of vectors

The polar sine changes sign whenever two vectors are interchanged, due to the antisymmetry of row-exchanging in the determinant; however, its absolute value will remain unchanged.

$\begin{array}{cc}\mathrm{\Omega }& =det\left[\begin{array}{cccccccc}{𝐯}_1& {𝐯}_2& \cdots & {𝐯}_i& \cdots & {𝐯}_j& \cdots & {𝐯}_n\end{array}\right]\\ & =-det\left[\begin{array}{cccccccc}{𝐯}_1& {𝐯}_2& \cdots & {𝐯}_j& \cdots & {𝐯}_i& \cdots & {𝐯}_n\end{array}\right]\\ & =-\mathrm{\Omega }\end{array}$

Invariance under scalar multiplication of vectors

The polar sine does not change if all of the vectors Template:Math are scalar-multiplied by positive constants Template:Math, due to factorization

$\begin{array}{cc}\mathrm{psin}(c_1{𝐯}_1,\dots ,c_n{𝐯}_n)& =\frac{det\left[\begin{array}{cccc}{c}_1{𝐯}_1& c_2{𝐯}_2& \cdots & c_n{𝐯}_n\end{array}\right]}{\prod _{i=1}^n\Vert c_i{𝐯}_i\Vert }\\ & =\frac{\prod _{i=1}^n{c}_i}{\prod _{i=1}^n|c_i|}\cdot \frac{det\left[\begin{array}{cccc}{𝐯}_1& {𝐯}_2& \cdots & {𝐯}_n\end{array}\right]}{\prod _{i=1}^n\Vert {𝐯}_i\Vert }\\ & =\mathrm{psin}({𝐯}_1,\dots ,{𝐯}_n).\end{array}$

If an odd number of these constants are instead negative, then the sign of the polar sine will change; however, its absolute value will remain unchanged.

Vanishes with linear dependencies

If the vectors are not linearly independent, the polar sine will be zero. This will always be so in the degenerate case that the number of dimensions Template:Mvar is strictly less than the number of vectors Template:Mvar.

Relationship to pairwise cosines

The cosine of the angle between two non-zero vectors is given by

$\cos ({𝐯}_1,{𝐯}_2)=\frac{{𝐯}_1\cdot {𝐯}_2}{\Vert {𝐯}_1\Vert \Vert {𝐯}_2\Vert }$

using the dot product. Comparison of this expression to the definition of the absolute value of the polar sine as given above gives:

${|\mathrm{psin}({𝐯}_1,\dots ,{𝐯}_n)|}^2=det\left[\begin{array}{cccc}1& \cos ({𝐯}_1,{𝐯}_2)& \cdots & \cos ({𝐯}_1,{𝐯}_n)\\ \cos ({𝐯}_2,{𝐯}_1)& 1& \cdots & \cos ({𝐯}_2,{𝐯}_n)\\ ⋮& ⋮& \ddots & ⋮\\ \cos ({𝐯}_n,{𝐯}_1)& \cos ({𝐯}_n,{𝐯}_2)& \cdots & 1\end{array}\right].$

In particular, for Template:Math, this is equivalent to

${\sin }^2({𝐯}_1,{𝐯}_2)=1-{\cos }^2({𝐯}_1,{𝐯}_2),$

which is the Pythagorean theorem.

History

Polar sines were investigated by Euler in the 18th century.

See also

• Trigonometric functions
• List of trigonometric identities
• Solid angle
• Simplex
• Law of sines
• Cross product and Seven-dimensional cross product
• Graded algebra
• Exterior derivative
• Differential geometry
• Volume integral
• Measure (mathematics)
• Product integral

External links

• Template:MathWorld