cr.sparse.opt.indicator_l1_ball(q=1.0, b=None, A=None)[source]

Returns an indicator function for the closed l1 ball \(\| A x - b \|_1 \leq q\)

  • q (float) – Radius of the ball

  • b (jax.numpy.ndarray) – A vector \(b \in \RR^{m}\)

  • A (jax.numpy.ndarray) – A matrix \(A \in \RR^{m \times n}\)


An indicator function

The indicator function is defined as:

(1)\[\begin{split}I(x) = \begin{cases} 0 & \text{if } \| A x - b \|_1 \leq q \\ \infty & \text{otherwise} \end{cases}\end{split}\]

Special cases:

  • indicator_l1_ball() returns the l1 unit ball \(\| x \|_1 \leq 1\).

  • indicator_l1_ball(q) returns the l1 ball \(\| x \|_1 \leq q\).

  • indicator_l1_ball(q, b=b) returns the l1 ball at center \(b\), \(\| x - b\|_1 \leq q\).


  • If center \(b \in \RR^m\) is unspecified, we assume the center to be at origin.

  • If radius \(q\) is unspecified, we assume the radius to be 1.

  • If the matrix \(A\) is unspecified, we assume \(A\) to be the identity matrix \(I \in \RR^{n \times n}\).