L1-regularized quadratic function

Definition

A $\ell ^{1}$-regularized quadratic function of one variable is a function of the form:

$f(x):=ax^{2}+bx+c+\lambda |x|$

where $a,b,c,\lambda \in \mathbb {R} ,a\neq 0,\lambda >0$.

The function has a piecewise quadratic function definition:

$f(x):=\left\lbrace {\begin{array}{rl}ax^{2}+(b+\lambda )x+c,&x\geq 0\\ax^{2}+(b-\lambda )x+c,&x<0\\\end{array}}\right.$

Differentiation

First derivative

The expression for the first derivative is obtained from the piecewise definition of the function, using the differentiation rule for piecewise definition by interval. We obtain the expression:

$f'(x)=\left\lbrace {\begin{array}{rl}2ax+(b+\lambda ),&x>0\\2ax+(b-\lambda ),&x<0\\\end{array}}\right.$

The first derivative is undefined at $x=0$. At this point, the left-hand derivative is $b-\lambda$ and the right-hand derivative is $b+\lambda$.

The derivative function is therefore a piecewise linear function with a jump discontinuity at zero.

Second derivative

The second derivative is defined as:

$f''(x)=2a,\qquad x\neq 0$

The second derivative is undefined at zero.

Points and intervals of interest

Critical points

As noted above, we have the following expression for the derivative:

$f'(x)=\left\lbrace {\begin{array}{rl}2ax+(b+\lambda ),&x>0\\2ax+(b-\lambda ),&x<0\\\end{array}}\right.$

We have at least one and at most three critical points. The three candidates are discussed below.

Critical point	Case that it occurs	Description of critical point
0	Always	The function is not differentiable at the point. The left-hand derivative is $b-\lambda$ and the right-hand derivative is $b+\lambda$.
${\frac {-(b+\lambda )}{2a}}$	The case that ${\frac {-(b+\lambda )}{2a}}>0$	It is the unique critical point for the quadratic piece for $x>0$. It occurs if and only if the critical point for that piece happens to fall within that piece definition.
${\frac {-(b-\lambda )}{2a}}$	The case that ${\frac {-(b-\lambda )}{2a}}<0$	It is the unique critical point for the quadratic piece for $x<0$. It occurs if and only if the critical point for that piece happens to fall within that piece definition.