# Least Squares Fitting

This note is more for myself than for anyone else. I have derived the expression for the least squares fitting so many times it’s not funny. The problem is, once I cobble together the routine to perform the fitting, I completely forget how to do it again. I hope, this will prevent me from having to do it ever again if only because it is on my website.

This post also gives me a chance to try out MathJax. After a Google search, I came across these instructions. Apparently, we simply need to add

<script type="text/javascript"
src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
</script>


to the layout. However, I don’t really want it in every page. So, I just put it at the top of this source. Oh while I’m at it, make sure you spell javascript right. If you don’t you could spend a few hours wondering what went wrong like I did when I spelled it javascrpit.

We begin with a set of function of the independent variable $$\{x_i\}$$ and dependent variables $$\{y_i\}$$. We then select a collection of functions to relate the two $y_i = a_0 +a_1 x_i +a_2 x_i^2 +\ldots +a_j \sin(x_i) =\sum_j a_j\,f_j(x_i).$ Now, we minimize the squared error $\frac{\partial}{\partial a_k} \frac{1}{N}\sum_i [y_i -\sum_j a_j\,f_j(x_i)]^2 = -\frac{2}{N} \sum_i [y_i -\sum_j a_j\,f_j(x_i)] f_k(x_i) = 0$ or in matrix form $\mathbf{a} \mathbf{F} \mathbf{F}^T = \mathbf{y} \mathbf{F}^T$ which can be readily solved for the coefficients $$\{a_j\}$$.

See, I told you that this was simple. Now to put this online and see how the math looks.

A few pointers:

• You must escape the backslashes in entering the math mode \$$… \$$ and \$… \$.
• The dollar sign version $$…$$ appears to work as inline math with kramdown.
• The \sum_j construct with no limits on the sum does not like with the index is inside {} unless you escape with a backslash (maybe. I didn’t actually test that).
Written on September 27, 2014