# Why l1 Norm is non-differentiable?

1. Jul 2, 2012

### venki1130

Can anyone explain Why l1 Norm is non-differentiable in terms of matrix calculus ?

2. Jul 3, 2012

### chogg

Because L1 norm is based on the absolute value of the difference, and absolute value |x| has a kink at x=0. It is not differentiable at the kink.

3. Jul 3, 2012

### algebrat

I believe venki1130 may have answered your question, but I am personally not sure. When you say l1 norm, do you mean norm of $(x_1,\dots,x_n)$ is $|x_1|+\cdots+|x_n|$? That is the first definition I found on wikipedia. I believe this is also called the taxicab metric.

If I try to recall my education, $\ell1$ and $L1$ are different, the first one is called little ell one. The second I believe is the integral version, $|f(x)|_1=\int|f(x)|dx$. Compare to $L2$, $|f(x)|_2=(\int|f(x)|^2dx)^{1/2}$. Little ell two, is $|(x_1,\dots,x_n)|_2=\sqrt{x_1^1+\cdots+x_n^2}$. This is sort of a distance as the crow flies, as opposed to how a taxi drives.

I believe the $\ell2$-norm has a familiar representation as a matrix, so that is what is confusing me. You asked for a matrix definition of $\ell1$-norm, when I only know of one for $\ell2$-norm.

Further, I could not tell you quickly how to use the matrix representation to show you the norm is not differentiable. I would guess that venki1130 pointed you in the right direction. In general, you could show it is not differentiable along any $x_i=0$ face. It would be easiest to check for $x_2=\cdots=x_n=0$, and $x_1$ near 0. In other words, show $|x_1|$ is not differentiable near zero. Simply care the slopes from the left and right of 0.

4. Jul 3, 2012

### micromass

Staff Emeritus
I think he means this: http://en.wikipedia.org/wiki/Matrix_norm

5. Jul 4, 2012

### venki1130

Thank you very much micromass, algebrat and Chogg for your response. I and using L1 norm in the optimization problem.

for example: For least squares optimization using L2 norm for regularization the equation I am using is

min ||Ax-b||22 + λ||x||22

Calculating first derivative(using matrix calculus) and equating it to zero results

x= At(AtA+λI)-1b

similarly for L1 norm

min ||Ax-b||22 + λ||x||1

But, People always say it is non differentiable. In fact, I understand the concept (intuitively, the unit circle in l1 has the sharp corner where the function doesn't change so there is no derivative for it) but I want to learn step by step using matrix derivatives.

Again I thank you very much for your help.

Venki