Change of Basis
Let V be a vector space and let S = {v1,v2,
, vn}
be a set of vectors in V. Recall that S forms a basis for
V if the following two conditions hold:
- S is linearly independent.
- S spans V.
If S = {v1,v2,
, vn} is a basis for V, then every
vector v ∈ V can be expressed uniquely as a linear
combination of v1,v2,
, vn:
v = c1v1 + c2v2 +
+ cnvn. |
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Think of |
/ | | | | | \ |
|
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as the coordinates of v relative to
the basis S. If V has dimension n, then every set of n
linearly independent vectors in V forms a basis for V. In every
application, we have a choice as to what basis we use. In this
tutorial, we will describe the transformation of coordinates of vectors
under a change of basis.
We will focus on vectors in R2, although all of this generalizes to
Rn. The standard basis in R2 is {[1 0]T,[0 1]T}. We specify other bases
with reference to this rectangular coordinate system.
Let B = {u,w} and B ′ = {u ′,w ′} be two bases for
R2. For a vector v ∈ V, given its coordinates [v]B in basis B we would like to be able to express v in
terms of its coordinates [v]B ′ in basis B ′, and vice versa.
Suppose the basis vectors u ′ and w ′ fo B ′ have the
following coordinates relative to the basis B:
The change of coordinates matrix from B ′ to B
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governs the change of coordinates of v ∈ V under the change
of basis from B ′ to B:
[v]B = P[v]B ′ = |
/ | \ |
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|
\ | / |
[v]B ′. |
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That is, if we know the coordinates of v relative to the basis
B ′, multiplying this vector by the change of coordinates matrix
gives us the coordinates of v relative to the basis B.
P viene chiamata anche matrice di transizione dalle nuove coordinate a quelle iniziali.
Why?
The transition matrix P is invertible. In fact, if P is the
change of coordinates matrix from B ′ to B, then P-1 is the
change of coordinates matrix from B to B ′:
Example
Let B = {[1 0]T,[0 1]T} and B ′ = {[3 1]T,[-2 1]T}. The change of basis matrix from B ′ to B is
The vector v with coordinates [v]B ′ = [2 1]T relative to the basis B ′ has coordinates
[v]B = |
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= |
/ | \ |
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relative to the basis B. Since
we can verify that
[v]B ′ = |
/ | \ |
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/ | \ |
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= |
/ | \ |
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which is what we started with.
In the following example, we introduce a third basis to look at the
relationship between two non-standard bases.
Example
Let B ′ ′ = |
{ |
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2
1
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, |
/ | \ |
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1
4
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} |
. |
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To find the change of coordinates matrix from the
basis B ′ of the previous example to B ′ ′, we first express the
basis vectors
of B ′ as linear combinations of the basis vectors
of
B ′ ′:
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11 7
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− |
1 7
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-9 7
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+ |
4 7
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a |
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+ b |
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c |
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+ d |
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and solve the resulting systems for a,b,c, and d.
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Thus, the transition matrix from B ′ to B ′ ′ is
|
/ | | | | | \ |
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. |
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The vector v with coordinates [2 1]T
relative to the basis B ′ has coordinates
|
/ | | | | | \ |
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/ | \ |
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= |
/ | | | | | \ |
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relative to the basis B ′ ′. This is, back in the standard basis,
[ v ]B = |
13 7
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/ | \ |
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+ |
2 7
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= |
/ | \ |
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, |
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which agrees with the results of the previous example.
Rotation of the Coordinate Axes
Suppose we obtain a new coordinate system from the standard
rectangular coordinate system by rotating the axes counterclockwise by
an angle θ. The new basis B ′ = {u ′, v ′}
of unit vectors along the x ′- and y ′-axes, respectively, has
coordinates
in the original coordinate system. Thus,
P = |
/ | \ |
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\ | / |
and P-1 = |
/ | \ |
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\ | / |
A vector [x y]TB in the
original coordinate system has coordinates [x ′ y ′]TB ′ given by
|
/ | \ |
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B ′
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= |
/ | \ |
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/ | \ |
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B
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in the rotated coordinate system.
Example
The vector [v]B = [3 2]T in the original
coordinate system has coordinates
[v]B ′ = |
/ | | | | | \ |
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/ | \ |
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= |
/ | | | | | \ |
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in the coordinate system formed by rotating the axes by 45°.
Note.
• Nel caso di una semplice rotazione degli assi si può osservare che P-1=Pt, ossia che l'inversa della matrice di transizione si può trovare semplicemente scambiando righe con colonne.
Una matrice A tale che A-1= At (ovvero
A×At=I) si dice ortogonale.
[dato che det(At)=det(A) e det(A-1)=1/det(A), si ha che det(A)=±1]
Si può dimostrare che ogni cambiamento di base in cui la nuova base e1,e2 sia ortonormale
(o cartesiana o euclidea),
ossia formata da vettori tra loro perpendicolari e di modulo 1 (ossia tali che ei·ej = 1 se i=j, ei·ej = 0 altrimenti)
è descritto da una matrice di transizione ortogonale (e viceversa: se la matrice di transizione
è ortogonale, da una base cartesiana si ottiene un'altra base cartesiana).
• Nel caso di R3 la rotazione degli assi è caratterizzata dagli angoli formati da ogni nuovo asse col corrispondente vecchio (vedi Geometria delle trasformazioni
lineari).
Nel caso di R2 P ha come prima colonna cos(θ),sin(θ) che equivale a cos(θ),cos(φ) (le
proiezioni del versore di x' sui vecchi assi) dove θ e φ (= π/2-θ) sono
gli angoli che x' forma con x e con y; e come seconda colonna
-sin(θ),cos(θ) che equivale a cos(ψ),cos(θ), dove
ψ (=π/2+θ) e θ sono gli angoli che y' forma con x e y. P-1 ha come elemento di posto (i,j) cos(x'i, xj),
avendo indicato i due assi con x1,x2.
Lo stesso accade per R3, dove x1,x2,x3 stanno per x,y,z
(P è la matrice A considerata alla fine di Geometria delle trasformazioni
lineari; P-1, per quanto osservato nella prima Nota, è la trasposta di A).
Key Concepts [index]
Let B = {u,v} and B ′ = { u ′, v ′ } be two bases for
R2. If [u]B = [a b]T and [v]B = [c d]T, then
is the change of coordinates matrix from
B ′ to B and P-1 is the change of coordinates matrix from B
to B ′. That is, for any v ∈ V,