- #1
unawareness
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Homework Statement
Show that the following linear transformation matrix is a contraction mapping.
\begin{bmatrix}
0.5 & 0 & -1 \\
0 & 0.5 & 1 \\
0 & 0 & 1
\end{bmatrix}
I don't know how to make that into a matrix, but it is a 3x3 matrix. The first row is [.5 0 -1] the second row is [0 .5 1] and the third row is [0 0 1].
Mod note: I formatted the matrix for you. If you quote this message, you can see how it's done.
Homework Equations
Well, I read some definitions. A function f is a contraction mapping if given a metric space (S,d) and a real constant 1 > c ≥ 0, then c [itex]\times[/itex] d(x,y) ≥ d(f(x),f(y)) [itex]\forall[/itex] x,y [itex]\in[/itex] S.
and
A metric space is an ordered pair (S,d) where S is a set and d is a metric (a notion of distance) on S.
d: S[itex]\times[/itex]S [itex]\mapsto[/itex] R
such that for any x,y,z [itex]\in[/itex] S the following holds:
1. d(x,y) ≥ 0
2. d(x,y) = 0 [itex]\Leftrightarrow[/itex] x = y
3. d(x,y) = d(x,y)
4. d(x,y) ≤ d(x,y) + d(y,z)
The Attempt at a Solution
Okay. I am almost certain it is a contraction mapping. The .5 makes me think that if an object is transformed with this matrix, it will shrink by a scale of 1/2 every time. I drew a little picture of an iterated mapping with a starting point (0,0) and saw that the composition of mappings converged to the point (-2,2). Each iteration got 1 + 1/2 the distance from the previous iteration to the point (-2,2). I don't know if that even means anything, though. I think that the third column of the matrix also implies a translation. I don't know how this plays into the contraction, though. I'm just lost in definitions. I might have "shown" it with my example, but I don't really know if I've shown anything because I'm clueless in general.
Also, in the definition of contraction mapping, I don't know what the "c" constant is. Like... in my specific problem would c = 1/2? If it is, why? I don't know how I could arbitrarily pull out the 1/2 in the matrix. Like... could it be a scalar 1/2 on a matrix with entries [1 0 -2] and so on for the other rows?
If c is 1/2, though, I could just pick two random points say x = 2 and y = 5 then use 1/2 for c and plug them all into the definition, and if it is satisfied, then the transformation would indeed be a contraction? The result does satisfy the definition as 1/2 [itex]\times[/itex] d(2,5) ≥ d(f(2),f(5)) = [itex]\sqrt{29}[/itex] / 2 ≥ [itex]\sqrt{29}[/itex] / 2, but I don't know if that process is correct in any way.
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