qiskit.algorithms.optimizers.L_BFGS_B
class L_BFGS_B(maxfun=1000, maxiter=15000, factr=10, iprint=- 1, epsilon=1e-08)
Limited-memory BFGS Bound optimizer.
The target goal of Limited-memory Broyden-Fletcher-Goldfarb-Shanno Bound (L-BFGS-B) is to minimize the value of a differentiable scalar function . This optimizer is a quasi-Newton method, meaning that, in contrast to Newtons’s method, it does not require ’s Hessian (the matrix of ’s second derivatives) when attempting to compute ’s minimum value.
Like BFGS, L-BFGS is an iterative method for solving unconstrained, non-linear optimization problems, but approximates BFGS using a limited amount of computer memory. L-BFGS starts with an initial estimate of the optimal value, and proceeds iteratively to refine that estimate with a sequence of better estimates.
The derivatives of are used to identify the direction of steepest descent, and also to form an estimate of the Hessian matrix (second derivative) of . L-BFGS-B extends L-BFGS to handle simple, per-variable bound constraints.
Uses scipy.optimize.fmin_l_bfgs_b. For further detail, please refer to https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.fmin_l_bfgs_b.html
Parameters
- maxfun (
int) – Maximum number of function evaluations. - maxiter (
int) – Maximum number of iterations. - factr (
float) – The iteration stops when (f^k - f^{k+1})/max{|f^k|, |f^{k+1}|,1} <= factr * eps, where eps is the machine precision, which is automatically generated by the code. Typical values for factr are: 1e12 for low accuracy; 1e7 for moderate accuracy; 10.0 for extremely high accuracy. See Notes for relationship to ftol, which is exposed (instead of factr) by the scipy.optimize.minimize interface to L-BFGS-B. - iprint (
int) – Controls the frequency of output. iprint < 0 means no output; iprint = 0 print only one line at the last iteration; 0 < iprint < 99 print also f and |proj g| every iprint iterations; iprint = 99 print details of every iteration except n-vectors; iprint = 100 print also the changes of active set and final x; iprint > 100 print details of every iteration including x and g. - epsilon (
float) – Step size used when approx_grad is True, for numerically calculating the gradient
__init__
__init__(maxfun=1000, maxiter=15000, factr=10, iprint=- 1, epsilon=1e-08)
Parameters
- maxfun (
int) – Maximum number of function evaluations. - maxiter (
int) – Maximum number of iterations. - factr (
float) – The iteration stops when (f^k - f^{k+1})/max{|f^k|, |f^{k+1}|,1} <= factr * eps, where eps is the machine precision, which is automatically generated by the code. Typical values for factr are: 1e12 for low accuracy; 1e7 for moderate accuracy; 10.0 for extremely high accuracy. See Notes for relationship to ftol, which is exposed (instead of factr) by the scipy.optimize.minimize interface to L-BFGS-B. - iprint (
int) – Controls the frequency of output. iprint < 0 means no output; iprint = 0 print only one line at the last iteration; 0 < iprint < 99 print also f and |proj g| every iprint iterations; iprint = 99 print details of every iteration except n-vectors; iprint = 100 print also the changes of active set and final x; iprint > 100 print details of every iteration including x and g. - epsilon (
float) – Step size used when approx_grad is True, for numerically calculating the gradient
Methods
__init__([maxfun, maxiter, factr, iprint, …]) | type maxfunint |
get_support_level() | Return support level dictionary |
gradient_num_diff(x_center, f, epsilon[, …]) | We compute the gradient with the numeric differentiation in the parallel way, around the point x_center. |
optimize(num_vars, objective_function[, …]) | Perform optimization. |
print_options() | Print algorithm-specific options. |
set_max_evals_grouped(limit) | Set max evals grouped |
set_options(**kwargs) | Sets or updates values in the options dictionary. |
wrap_function(function, args) | Wrap the function to implicitly inject the args at the call of the function. |
Attributes
bounds_support_level | Returns bounds support level |
gradient_support_level | Returns gradient support level |
initial_point_support_level | Returns initial point support level |
is_bounds_ignored | Returns is bounds ignored |
is_bounds_required | Returns is bounds required |
is_bounds_supported | Returns is bounds supported |
is_gradient_ignored | Returns is gradient ignored |
is_gradient_required | Returns is gradient required |
is_gradient_supported | Returns is gradient supported |
is_initial_point_ignored | Returns is initial point ignored |
is_initial_point_required | Returns is initial point required |
is_initial_point_supported | Returns is initial point supported |
setting | Return setting |
bounds_support_level
Returns bounds support level
get_support_level
get_support_level()
Return support level dictionary
gradient_num_diff
static gradient_num_diff(x_center, f, epsilon, max_evals_grouped=1)
We compute the gradient with the numeric differentiation in the parallel way, around the point x_center.
Parameters
- x_center (ndarray) – point around which we compute the gradient
- f (func) – the function of which the gradient is to be computed.
- epsilon (float) – the epsilon used in the numeric differentiation.
- max_evals_grouped (int) – max evals grouped
Returns
the gradient computed
Return type
grad
gradient_support_level
Returns gradient support level
initial_point_support_level
Returns initial point support level
is_bounds_ignored
Returns is bounds ignored
is_bounds_required
Returns is bounds required
is_bounds_supported
Returns is bounds supported
is_gradient_ignored
Returns is gradient ignored
is_gradient_required
Returns is gradient required
is_gradient_supported
Returns is gradient supported
is_initial_point_ignored
Returns is initial point ignored
is_initial_point_required
Returns is initial point required
is_initial_point_supported
Returns is initial point supported
optimize
optimize(num_vars, objective_function, gradient_function=None, variable_bounds=None, initial_point=None)
Perform optimization.
Parameters
- num_vars (int) – Number of parameters to be optimized.
- objective_function (callable) – A function that computes the objective function.
- gradient_function (callable) – A function that computes the gradient of the objective function, or None if not available.
- variable_bounds (list[(float, float)]) – List of variable bounds, given as pairs (lower, upper). None means unbounded.
- initial_point (numpy.ndarray[float]) – Initial point.
Returns
point, value, nfev
point: is a 1D numpy.ndarray[float] containing the solution value: is a float with the objective function value nfev: number of objective function calls made if available or None
Raises
ValueError – invalid input
print_options
print_options()
Print algorithm-specific options.
set_max_evals_grouped
set_max_evals_grouped(limit)
Set max evals grouped
set_options
set_options(**kwargs)
Sets or updates values in the options dictionary.
The options dictionary may be used internally by a given optimizer to pass additional optional values for the underlying optimizer/optimization function used. The options dictionary may be initially populated with a set of key/values when the given optimizer is constructed.
Parameters
kwargs (dict) – options, given as name=value.
setting
Return setting
wrap_function
static wrap_function(function, args)
Wrap the function to implicitly inject the args at the call of the function.
Parameters
- function (func) – the target function
- args (tuple) – the args to be injected
Returns
wrapper
Return type
function_wrapper