# Least square regression with L1 regularization and non-negativity constraint

There are two functions associated by the model

$a(x) = \int_{k_1}^{k_2} b(k)\exp(-kx)dk$

where $a(x)$ is the experimental data I have, and $b(k)$ is the information I want to get. Or I can write in a matrix form if k space is discretized:

$a = Mb$

Now I would like to solve this inverse problem with L1 regularization with b>=0 constrain:

min: $|Mb-a|^2 + \lambda|b|$

subject to: $b\geq 0$

with $\lambda$ as the regularization parameter.

(Actually this problem is similar to a previous one, but this time it is L1 norm regularization.)

My questions are:

1. While general solvers for the unconstrained least square with L1 regularization problems exist, I have some difficulties finding a solver for the constrained case like here. Is there an existing solver/routine I can make use of? If not, is there an easy way to get on this problem?
2. I don't know the noise level. Someone suggested cross-validation method to get the regularization parameter. I have some difficulties understanding this method. Could anyone give an easy explanation and point me to the appropriate resources for further understanding?

Update: regarding the first question above, I have found this Matlab solver is really great.

• Sorry, I should have clarified my previous comment. I'm reposting it with explanation. The link to Rob Hyndman's post on Cross-validation was in answer to Q2 – Glen_b Feb 25 '14 at 5:04
• To clarify, that post's discussion of cross validation gives what is alternatively known as the "PRESS" statistic or the "deleted residual," which uses the hat matrix as a shortcut to doing leave-one-out cross validation. If you're not satisfied with the explanation in Glen's link, try googling those terms for more information. – David Marx Feb 25 '14 at 5:14
• Thanks. I just read the post link. However, for the non-negativity constrained L1 problem I have, can I still use the form of H matrix (described in the "Cross-​​validation for lin­ear models" section of Glen's link) for the purpose of generalized cross-validation? It seems that there should be some way to include the regularization parameter $\lambda$, and I cannot see there is a closed form of it for a given $\lambda$. – shva Feb 25 '14 at 21:21