# pylops.optimization.cls_sparsity.SPGL1¶

class pylops.optimization.cls_sparsity.SPGL1(Op, callbacks=None)[source]

Solve a constrained system of equations given the operator Op and a sparsyfing transform SOp aiming to retrive a model that is sparse in the sparsyfing domain.

This is a simple wrapper to spgl1.spgl1 which is a porting of the well-known SPGL1 MATLAB solver into Python. In order to be able to use this solver you need to have installed the spgl1 library.

Parameters: Op : pylops.LinearOperator Operator to invert of size $$[N \times M]$$. ModuleNotFoundError If the spgl1 library is not installed

Notes

Solve different variations of sparsity-promoting inverse problem by imposing sparsity in the retrieved model .

The first problem is called basis pursuit denoise (BPDN) and its cost function is

$\|\mathbf{x}\|_1 \quad \text{subject to} \quad \left\|\mathbf{Op}\,\mathbf{S}^H\mathbf{x}-\mathbf{y}\right\|_2^2 \leq \sigma,$

while the second problem is the ℓ₁-regularized least-squares or LASSO problem and its cost function is

$\left\|\mathbf{Op}\,\mathbf{S}^H\mathbf{x}-\mathbf{y}\right\|_2^2 \quad \text{subject to} \quad \|\mathbf{x}\|_1 \leq \tau$
  van den Berg E., Friedlander M.P., “Probing the Pareto frontier for basis pursuit solutions”, SIAM J. on Scientific Computing, vol. 31(2), pp. 890-912. 2008.

Methods

 __init__(Op[, callbacks]) Initialize self. callback(x, *args, **kwargs) Callback routine finalize(*args[, show]) Finalize solver run(x[, show]) Run solver setup(y[, SOp, tau, sigma, show]) Setup solver solve(y[, x0, SOp, tau, sigma, show]) Run entire solver step() Run one step of solver
setup(y, SOp=None, tau=0, sigma=0, show=False)[source]

Setup solver

Parameters: y : np.ndarray Data of size $$[N \times 1]$$ SOp : pylops.LinearOperator, optional Sparsifying transform tau : float, optional Non-negative LASSO scalar. If different from 0, SPGL1 will solve LASSO problem sigma : list, optional BPDN scalar. If different from 0, SPGL1 will solve BPDN problem show : bool, optional Display setup log
step()[source]

Run one step of solver

This method is used to run one step of the solver. Users can change the function signature by including any other input parameter required when applying one step of the solver

Parameters: x : np.ndarray Current model vector to be updated by a step of the solver show : bool, optional Display step log
run(x, show=False, **kwargs_spgl1)[source]

Run solver

Parameters: x : np.ndarray Current model vector to be updated by multiple steps of the solver. If None, x is assumed to be a zero vector show : bool, optional Display iterations log **kwargs_spgl1 Arbitrary keyword arguments for spgl1.spgl1 solver xinv : numpy.ndarray Inverted model in original domain. pinv : numpy.ndarray Inverted model in sparse domain. info : dict Dictionary with the following information: tau, final value of tau (see sigma above) rnorm, two-norm of the optimal residual rgap, relative duality gap (an optimality measure) gnorm, Lagrange multiplier of (LASSO) stat, final status of solver 1: found a BPDN solution, 2: found a BP solution; exit based on small gradient, 3: found a BP solution; exit based on small residual, 4: found a LASSO solution, 5: error, too many iterations, 6: error, linesearch failed, 7: error, found suboptimal BP solution, 8: error, too many matrix-vector products. niters, number of iterations nProdA, number of multiplications with A nProdAt, number of multiplications with A’ n_newton, number of Newton steps time_project, projection time (seconds) time_matprod, matrix-vector multiplications time (seconds) time_total, total solution time (seconds) niters_lsqr, number of lsqr iterations (if subspace_min=True) xnorm1, L1-norm model solution history through iterations rnorm2, L2-norm residual history through iterations lambdaa, Lagrange multiplier history through iterations
solve(y, x0=None, SOp=None, tau=0, sigma=0, show=False, **kwargs_spgl1)[source]

Run entire solver

Parameters: y : np.ndarray Data of size $$[N \times 1]$$ x0 : numpy.ndarray, optional Initial guess SOp : pylops.LinearOperator, optional Sparsifying transform tau : float, optional Non-negative LASSO scalar. If different from 0, SPGL1 will solve LASSO problem sigma : list, optional BPDN scalar. If different from 0, SPGL1 will solve BPDN problem show : bool, optional Display log **kwargs_spgl1 Arbitrary keyword arguments for spgl1.spgl1 solver xinv : numpy.ndarray Inverted model in original domain. pinv : numpy.ndarray Inverted model in sparse domain. info : dict Dictionary with the following information: tau, final value of tau (see sigma above) rnorm, two-norm of the optimal residual rgap, relative duality gap (an optimality measure) gnorm, Lagrange multiplier of (LASSO) stat, final status of solver 1: found a BPDN solution, 2: found a BP solution; exit based on small gradient, 3: found a BP solution; exit based on small residual, 4: found a LASSO solution, 5: error, too many iterations, 6: error, linesearch failed, 7: error, found suboptimal BP solution, 8: error, too many matrix-vector products. niters, number of iterations nProdA, number of multiplications with A nProdAt, number of multiplications with A’ n_newton, number of Newton steps time_project, projection time (seconds) time_matprod, matrix-vector multiplications time (seconds) time_total, total solution time (seconds) niters_lsqr, number of lsqr iterations (if subspace_min=True) xnorm1, L1-norm model solution history through iterations rnorm2, L2-norm residual history through iterations lambdaa, Lagrange multiplier history through iterations