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jeku1987
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Homework Statement
This isn't quite homework (though it's close), but I'm working on a quantum circuit and wish to implement an iterative method to find the square root of a number using elementary arithmetic operations (and without reciprocals). I've gotten most of it down, but the error analysis is kicking me hard in the teeth.
Homework Equations
I want to estimate the error of the following iteration that finds x:
Take [itex]x^2=a[/itex] where a is known (sort of; the QC would be calculating a bunch of possible a's in superposition via the inverse quantum Fourier transform/eigenvalue kickback, but that's not super relevant) and [itex]0< a <1[/itex], then make the substitutions of [itex]y=1-x[/itex] and [itex]b=1-a[/itex] to get [itex]2y=(b+y^2)[/itex]. You can turn this into [itex]y_{i+1}=\frac{1}{2}(b+y_{i}^{2})[/itex] which converges monotically, and converges for any [itex]y_{0}[/itex] between 0 and 1. So far, we're gravy.
The Attempt at a Solution
Now if we subtract y from both sides, we get
[tex]y_{i+1} - y = \frac{1}{2}(y_{i}^{2} - y^{2})[/tex]
And after some manipulation, along with some blindly-added absolute value signs, we end up with
[tex]\left | y_{i+1}-y \right | = \frac{1}{2} \left | y_{i}-y \right | \left | y_{i}+y \right |[/tex]
Here's the meat of my issue; I want [itex]\left | y_{i+1}-y \right | < \varepsilon[/itex] and to get there my intuition says to take some delta such that [itex]\frac{1}{2}\left | y_{0}-y \right | < \delta[/itex] which would lead to [itex]\left | y_{i}-y \right | < \delta^i\left | y_{0}-y \right | [/itex]
But at this point I'm kind of getting lost in all of the details of the individual parts and can't really step outside very well to see the Big Picture. How can I represent the error of the iterations in terms of things that I know (which in this case, I think is only going to be [itex]a[/itex])? And I don't know where to go from here, is anyone well-versed in error estimation? Or analysis? Am I done here, or are there more steps to go?
Any and all help is, of course, immensely appreciated.