Notations
Einstein summation convention is used here. A matrix M is denoted as and its ij-th element is referred to by
. Quantities or coefficients are indexed as for example
,
or
. These indices do not automatically pertain to row and column indices of a matrix, but the quantities can be presented by matrices through isomorphisms once their indices are freely interpreted as rows and columns of matrices.
Coordinates of a vector
Let be a n-dimensional vector space and
with
be a basis for
. Then, we define the coordinate function as,
such that for a vector written by its components (with respect to
) as
the function acts as,
The coordinate function is a linear map.
Change of basis for vectors
Let and
be two basis for
, then,
and
where the indices of the scalar terms and
are intentionally set this way. So, if all
are collected into a matrix
, then the sum
is over the rows of the matrix for a particular column. In other words, we can utilize the rule of matrix multiplication and write,
The same is true for . In above formulations, note that
is a dummy index (i.e. we can equivalently write
)
Setting as the initial (old) basis and writing the current (new) basis
in terms of
is referred to as forward transform denoted by
. Relatively,
is called backward transform.
The relation between that forward and backward transforms is obtained as follows,
We now find how vector coordinates are transformed relative to different bases. A particular can be expressed by its components according to any of
or
basis, therefore,
To find the relation between and
we write,
As it can be observed, the old basis to new basis is transformed by the forward transform while the old coordinates
are transformed to the new ones,
, by the backward transform
. Because the coordinates of
behave contrary to the basis vectors in transformation, the coordinates or the scalar components are said to be contravariant. A vector can be called a contravariant object because its scalar components (coordinates) transforms differently from the basis vectors whose their linearly combination equals to the vector. Briefly,
Proposition: Let . Then, the scalar components/coordinates
are transformed by
if and only if the basis vectors
are transformed by
, such that
.
Later, a vector is called a contravariant tensor. For the sake of notation and to distinguish between the transformations of the basis and the coordinates of a vector, in index of a coordinate is written as superscript to show it is contravariant. Therefore,
Linear maps and linear functionals
Definition: is defined as the space of all linear maps
where the domain and codomain are vectors spaces.
It can be proved that is a vector space
, hence, for
and
Note that the addition on the LHS is an operator in and the addition on the RHS is an operator in
.
Proposition 1: Let , i.e a linear map from a vector space
to another one
. If
is a basis for
, and
for
and
, then
is uniquely defined over
.
This proposition says a linear map over a space is uniquely determined by its action on the basis vectors of that space. In other words, if and
then
. proof: let
(given by the nature of
), then for
such that
, we can write
, therefore,
. Because,
‘s are unique for (a particular)
then
is unique for
and hence
must be unique for any
. In other word, there is only one unique
over
such that
.
As a side remark, if is a basis for
, hence spanning
, then
spans the range of
; The range of
is a subset of
.
By this proposition, a matrix completely determining a linear can be obtained for the linear map. let be n-dimensional with a basis
, and
be m-dimensional with a basis
. Then there are coefficients
such that,
In the notation , the index
is superscript because for a fixed
and hence a fixed
, the term
is the coordinate of
and it is a contravariant (e.g
).
For , and
, with the coordinates
and
, we write,
This expression can be written as a matrix multiplication of , where
presented by its elements as,
As a remark, above can be viewed as columns of the matrix and written as,
Linear functional (linear form or covector)
Definition: a linear functional on is a linear map
. The space
is called the dual space of
.
Proposition: Let and
be defined as
. Then,
called dual basis of
, is a basis of
, and hence
.
Proof: first we show that ‘s are linearly independent, i.e.
. Note that on the RHS,
. For a
we can write
and assume
. Then,
Since
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Now we prove that spans
. I.e
such that
. To this end, we apply both sides to a basis vector of
and write
which implies
or explicitly
is found as
. Consequently,
■.
Consider and
. If
, then the matrix of the linear functional/map
is
So, for as
we can write,
Result: if the coordinates of a vector is shown by a column vector or single-column matrix (which is a vector in the space of ), then a row vector or a single-row matrix represents the matrix of a linear functional.
Definition: a linear functional , which can be identified with a row vector as its matrix, is also called a covector.
Like vectors, a covector (and any linear map) is a mathematical object that is independent of a basis (i.e. invariant). The geometric representation of a vector in (or by an isomorphism in) is an arrow in
. For a covector isomorphic to
, the representation is a set (stack) of planes in
that can be represented by iso lines in
. A covector that is isomorphic to
can be represented by iso surfaces in
.
Example: Let be a basis of
and
be the matrix of a covector
in some
. Then, if
, we can write,
which, for different values of , is a set of (iso) lines in a Cartesian CS defined by two axes
and
along
and
that are the geometric representations of
and
. The Cartesian axes are not necessarily orthogonal.
If we chose any other basis for
, then the matrix of the covector
changes. Also, the geometric representations of
are different from
and
and hence the geometric representation of the covector stays the same shape.
Example: Let be a basis of
and
be a basis for
. This means
and
. Then, the matrix of each dual basis vector is as,
Change of basis for covectors
Let and
be two bases for
, and hence,
and
be two bases for
. Each dual basis vector
can be written in terms of the (old) dual basis vectors by using a linear transformation as
. Now, the coefficients
are to be determined as follows,
Using the formula regarding the change of basis of vectors, the above continues as,
This indicates that the dual basis are transformed by the backward transformation. Referring to the index convention, we use superscript for components that are transformed trough a backward transformation. Therefore,
meaning that dual basis vectors are contravariant because they behave contrary to the basis vectors in transformation from to
.
Now let . Writing
and using the above relation, we get,
meaning that they are transforming in a covariant manner when the basis of the vector space changes from to
.
Briefly the following relations have been shown.
![](https://bearishnotes.com/wp-content/uploads/tensor1_3621.jpg)
Basis and change of basis for the space of linear maps ![Rendered by QuickLaTeX.com \mathcal L(V, W)](https://bearishnotes.com/wp-content/ql-cache/quicklatex.com-47f953d8ee689a75fcacd0b67381274b_l3.png)
As can be proved is a linear vector space and any linear map is a vector. Therefore, we should be able to find a basis for this space. If
is n-dimensional and
is m-dimensional, the
is mn-dimensional and hence its basis should have
vectors, i.e. linear maps. Let’s enumerate the basis vectors of
as
for
and
, then any linear map
can be written as,
By proposition 1, any linear map is uniquely determined by its action on the basis vectors of its codomain. If be a basis for
, then for any basis vector
,
Setting a basis for as
, the above equation becomes,
Note that s are dummy indices. This equation holds if,
Therefore, we can choose a set of basis vectors
for
as,
By recruiting the basis of , the above can be written as,
The term is obviously a linear map
. It can be readily shown that
being a linear combinations of the derived basis vectors is linearly independent, i.e.
for any
(here, note that
).
So, a linear map can be written as a linear combination
. Here, it is necessary to use the index level convention. To this end, we observe that for a fixed
the term
couples with
and represents the coordinates of a covector. As coordinates of a covector are covariant, index
is written as subscript. For a fixed
though, the term
couples with
and represents the coordinates of a vector. As coordinates of a vector are contravariant, index
should rise. Therefore, we write,
The coefficients can be determined as,
Stating that are the coordinates of
with respect to the basis of
, i.e.
. Comparing with what was derived as
, we can conclude that
. Therefore,
The above result can also be derived from as follows.
Change of basis of is as follows.
For , let
and
be bases for
, and
and
be bases for
. Also,
and
are corresponding bases of
. Forward and backward transformation pairs in
and
are denoted as
and
.
Note that the coordinates
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![Rendered by QuickLaTeX.com s](https://bearishnotes.com/wp-content/ql-cache/quicklatex.com-1edc883862ceed1a21913f60358e31d8_l3.png)
![Rendered by QuickLaTeX.com T_s^k](https://bearishnotes.com/wp-content/ql-cache/quicklatex.com-8971b14c028310f6ba985ff8936f5b94_l3.png)
![Rendered by QuickLaTeX.com k](https://bearishnotes.com/wp-content/ql-cache/quicklatex.com-d42bc2203d6f76ad01b27ac9acc0bee1_l3.png)
Example: let , then,
If the matrices, ,
, and
are considered, we can write,
Bilinear forms
A bilinear form is a bilinear map defined as . Setting a basis for
, a bilinear form can be represented by matrix multiplications on the coordinates of the input vectors. If
is a basis for
, then
which can be written as,
where with
.
The expression indicates that a bilinear form is uniquely defined by its action on the basis vectors. This is the same as what was shown for linear maps by proposition 1. This comes from the fact that a bilinear form is a linear map with respect to one of its arguments at a time.
Now we seek a basis for the space of bilinear forms, i.e. . This is a vector space with the following defined operations.
The dimension of this space is , therefore, for any bilinear form
there are bilinear forms
such that,
From the result we can conclude that
Following the index level convention, the indices of should stay as subscripts because each index pertains to the covariant coordinates of a covector after fixing the other index.
If and
are two bases for
, then the change of basis of the space of bilinear forms are as follows.
Example: the metric bilinear map (metric tensor)
Dot/inner product on the vector space over
is defined as a bilinear map
such that,
and
. With this regard two objects (that can have geometric interpretations for Euclidean spaces) are defined as,
1- Length of a vector
2- Angle between two vectors
Let see how the dot product is expressed through the coordinates of vectors. With being a basis for
, we can write,
The term is called the metric tensor and its components can be presented by an n-by-n matrix as
.
If the basis is an orthonormal basis, i.e. , then
and
is the identity matrix. Therefore,
and
.
Multilinear forms
A general multilinear form is a multilinear map defined as , where
is a vector space. Particularly setting
leads to a simpler multilinear form as
.
Following the same steps as shown for a bilinear map, a multilinear form can be written as,
which implies,
showing that a multilinear form can be written as a linear combination of covectors.
Multilinear maps
A multilinear map is a map . A multilinear map can be written in terms of vector and covector basis. For example, consider
as
with
and
being bases for
and
. We can write,
Because for each and
we have
, we can write,
We write the indices and
in
as subscripts in accordance with their position on the LHS; however, we’ll see that
is a coordinate of a covector for each
or
when
is fixed. Combining above, we can write,
The term collects
coefficients and uniqully defines the multilinear map. We can imagine this term as a 3-dimensional array/matrix. Above also shows that the multilinear map can be written as a linear combination of basis covectors and basis vectors.
Definition of a tensor
Defining the following terms,
- Vector space
and basis
and another basis
.
- Basis transformation as
, and therefore
.
- The dual vector space of
as
.
- Vector space
and basis
and another basis
.
- Basis transformation as
, and therefore
- Linear map
.
- Bilinear form
.
we concluded that,
It is observed that if a vector is written in terms a single sum/linear combination of basis vectors of
, then the components of the vectors change contravariantely with respect to a change of basis. Then, the covectors are considered and it is observed that their components change covariently upon change of basis of
or
. A linear map can be written as a linear combination of vectors and covectors. The coefficients of this combination is seen to change both contra- and covariantely when the bases (of
and
) change. A bilinear form though can be written in terms of a linear combination of covectors. The corresponding coefficients change covariantly with change of basis. These results can be generalized toward an abstract definition of a mathematical object called a tensor. There are two following approaches for algebraically denfining a tensor.
Tensor as a multilinear form
Motivated by how linear maps, bilinear forms, and multilinear forms and maps can be written by combining basis vectors and covectors, a generalized combination of these vectors can considered. For example,
This object consists of a linear combination of a unified (merged) set of basis vector and covectors
(of
and
) by scalar coefficients
. According to the type of the basis vectors, the indices become sub- or superscript, and hence it determines the type of the transformation regarding that index. By reordering the basis vectors and covectors, we can write,
Recalling that vector components can be written as implines that there is a map
for a particular vector
such that,
And also the components of a covector determined as,
motivates defining as the array (collection of the coefficients) of a multilinear form,
Therefore, the object or the multi-dimensional array which is dependent on chosen bases of
and
and its transformation rules are based on the types of the beases (or indices) can be intrinsically related to an underlying multilineat map
. By virtue of this observation, an object called a tensor is defined as the following.
Definition: A tensor of type (r,s) on a vector space over a field
(or
) is a multilinear map as,
The coordinates or the (scalar) components of a tensor can then be determined once a basis
for
and a basis
for
are fixed. Therefore,
Note that r is the number of covariant indices and s is the number of contravariant indices. A tensor of type (r,s) can be imagined as an r+s-dimensional array of data containing elements. Each index corresponds to a dimention of the data array.
By this definition, a vector is a (0,1) tensor as it can be viewed as,
This implies that for each there is a (multilinear with one input) map
receiving a basis covector and returning the corresponding scalar component of the vector (
). This corresponding map is unique for each vector and it is called a tensor.
A covector (dual vector or a linear form) is a (1,0) tensor because a covector
is a linear map
َ A linear map (or
) is a (1,1) tensor because the term
in
pertains to a covector’s components for a fixed
and to a vector’s components for a fixed
. Therefore,
in other words, a linear map is a tensor viewed as a multilinear map . Here, the multilibear form can be considered as
by which gives
. Note that
returns a vector and
being
extract its j-th coordinate, and hencem the array/matrix of the linear map is retrieved.
A bilinear form is then a (2,0) tensor, where
A multilinear form is a (2,1) tensor where
where
can be considered as
.
As an example, the cross product of two vectors defined as
is a multilinear map
is a (2,1) tensor.
By convension, scalars are (0,0) tensors.
Remark: for a tensor we can write,
Example: Stress tensor. The Cauchy stress tensor in mechanics is a linear map and hence a (1,1) tensor.
Rank of a tensor
The rank of a (r,s)-type tensor is defined as r+s. In this regard, tensors of different types can have the same rank. For example tensors of types (1,1), (2,0), (0,2) have the same rank being 2. Here we compare these tensors with eachother.
A (1,1) tensor representing a linear map is where
with
.
A (2,0) tensor representing a bilinear form is where
with
.
A (0,2) tensor is where
with
.
The coeffieints of the above tensor are collected in 2-dimensional array/matrix; however, they follow different transformation rules based on their types.