The Inverse Function Theorem
Recall that if a coordinate transformation T maps an open region U in the uv-plane to
an open region V in the xy-plane, then T is 1-1 if each point in V is the image of only one point in U.
Additionally, if every point in V is the image under T(u,v) of at
least one point in U, then T(u,v) is said to map U onto V.
If T( u,v) is a 1-1 mapping of a region U in the uv-plane
onto a region V in the xy-plane, then we define the inverse
transformation of T from V onto U by
T-1( x,y) = ( u,v) only if (x,y) = T( u,v) |
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The Jacobian determinant can be used to determine if T has an inverse
transformation T-1 on at least some small region about a given point.
Inverse Function Theorem: Let T( u,v) be a coordinate
transformation on an open region S in the uv-plane and let (p,q) be a point in S. If
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¶( x,y)
¶( u,v)
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ê ê
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( u,v) = ( p,q)
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¹ 0 |
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then there is an open region U containing ( p,q) and an open
region V containing ( x,y) = T( p,q) such that T-1 exists and maps V onto U.
The proof of the inverse function theorem follows from the fact that the Jacobian matrix of T-1(x,y), if it exists, is given by the inverse of the
Jacobian of T,
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J-1( x,y) = |
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¶( x,y)
¶( u,v) |
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-1
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é ê
ë
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ù ú
û
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which features a Jacobian determinant with a negative power (see the exercises).
Thus, J-1 exists only if the
determinant of J(u,v) is non-zero.
EXAMPLE 7 Where is T( r,q) =
á r cos(q), r sin(q)
ñ
invertible?
Solution: The Jacobian determinant for polar coordinates is
which is non-zero everywhere except the origin. Thus, at any point (r0,q0) with r0 > 0, there is an open region U in
the rq-plane and an open region V containing ( x,y) = ( r0cos( q0) ,r0sin( q0) ) such that T-1( x,y) exists and maps V onto U.
We will explore the result in example 7 more fully in the
exercises. In particular, we will show that
Clearly, T-1 is not defined on any open region containing (0,0) . Also, if y = 0 and x > 0, then
2tan-1 |
æ ç
è
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y
x+ |
 |
 |
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x2+02 |
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ö ÷
ø
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= 2tan-1 |
æ è
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0
x+ |x|
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ö ø
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= 0 |
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But if y = 0 and x < 0, then
2tan-1 |
æ ç
è
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y
x+ |
 |
 |
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x2+y2 |
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ö ÷
ø
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= 2tan-1 |
æ è
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0
x+| x|
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ö ø
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= 2tan-1 |
æ è
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0
0
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ö ø
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That is, a different representation of T-1 must be used on any region which intersects the negative real axis.