In the study of
geometric algebras, a -blade or a simple -vector is a generalization of the concept of
scalars
Scalar may refer to:
* Scalar (mathematics), an element of a field, which is used to define a vector space, usually the field of real numbers
* Scalar (physics), a physical quantity that can be described by a single element of a number field such ...
and
vectors to include ''simple''
bivector In mathematics, a bivector or 2-vector is a quantity in exterior algebra or geometric algebra that extends the idea of scalars and vectors. If a scalar is considered a degree-zero quantity, and a vector is a degree-one quantity, then a bivector ca ...
s,
trivectors, etc. Specifically, a -blade is a
-vector that can be expressed as the
exterior product (informally ''wedge product'') of 1-vectors, and is of ''
grade
Grade most commonly refers to:
* Grade (education), a measurement of a student's performance
* Grade, the number of the year a student has reached in a given educational stage
* Grade (slope), the steepness of a slope
Grade or grading may also ref ...
'' .
In detail:
*A 0-blade is a
scalar
Scalar may refer to:
*Scalar (mathematics), an element of a field, which is used to define a vector space, usually the field of real numbers
*Scalar (physics), a physical quantity that can be described by a single element of a number field such a ...
.
*A 1-blade is a
vector. Every vector is simple.
*A 2-blade is a ''simple''
bivector In mathematics, a bivector or 2-vector is a quantity in exterior algebra or geometric algebra that extends the idea of scalars and vectors. If a scalar is considered a degree-zero quantity, and a vector is a degree-one quantity, then a bivector ca ...
. Sums of 2-blades are also bivectors, but not always simple. A 2-blade may be expressed as the wedge product of two vectors and :
*:
*A 3-blade is a simple trivector, that is, it may be expressed as the wedge product of three vectors , , and :
*:
*In a
vector space of
dimension , a blade of grade is called a ''
pseudovector
In physics and mathematics, a pseudovector (or axial vector) is a quantity that is defined as a function of some vectors or other geometric shapes, that resembles a vector, and behaves like a vector in many situations, but is changed into its ...
''
or an ''
antivector An antivector is an element of grade in an ''n''-dimensional exterior algebra. An antivector is always a blade, and it gets its name from the fact that its components each involve a combination of all except one basis vector, thus being the oppos ...
''.
*The highest grade element in a space is called a ''
pseudoscalar'', and in a space of dimension is an -blade.
*In a vector space of dimension , there are dimensions of freedom in choosing a -blade, of which one dimension is an overall scaling multiplier.
[For Grassmannians (including the result about dimension) a good book is: . The proof of the dimensionality is actually straightforward. Take vectors and wedge them together and perform elementary column operations on these (factoring the pivots out) until the top block are elementary basis vectors of . The wedge product is then parametrized by the product of the pivots and the lower block. Compare also with the dimension of a Grassmannian, , in which the scalar multiplier is eliminated.]
A
vector subspace of finite dimension may be represented by the -blade formed as a wedge product of all the elements of a basis for that subspace.
Indeed, a -blade is naturally equivalent to a -subspace endowed with a
volume form (an alternating -multilinear scalar-valued function) normalized to take unit value on the -blade.
Examples
In two-dimensional space, scalars are described as 0-blades, vectors are 1-blades, and area elements are 2-blades in this context known as
pseudoscalars, in that they are elements of a one-dimensional space distinct from regular scalars.
In three-dimensional space, 0-blades are again scalars and 1-blades are three-dimensional vectors, while 2-blades are oriented area elements. In this case 3-blades are called pseudoscalars and represent three-dimensional volume elements, which form a one-dimensional vector space similar to scalars. Unlike scalars, 3-blades transform according to the
Jacobian determinant of a
change-of-coordinate function.
See also
*
Grassmannian
*
Multivector
*
Exterior algebra
*
Differential form
*
Geometric algebra
*
Clifford algebra
In mathematics, a Clifford algebra is an algebra generated by a vector space with a quadratic form, and is a unital associative algebra. As -algebras, they generalize the real numbers, complex numbers, quaternions and several other hypercom ...
Notes
References
*
*
A Lasenby, J Lasenby & R Wareham(2004) ''A covariant approach to geometry using geometric algebra'' Technical Report. University of Cambridge Department of Engineering, Cambridge, UK.
*{{cite book , title=Computer algebra and geometric algebra with applications , year=2005 , page=329 ''ff'' , author=R Wareham , author2=J Cameron , author3=J Lasenby , name-list-style=amp , chapter=Applications of conformal geometric algebra to computer vision and graphics , chapter-url=https://books.google.com/books?id=uxofVAQE3LoC&pg=PA330 , editor1= Hongbo Li, editor2=Peter J Olver, editor2-link=Peter J. Olver, editor3=Gerald Sommer , isbn=3-540-26296-2 , publisher=Springer
External links
A Geometric Algebra Primer especially for computer scientists.
Geometric algebra
Vector calculus