# quasinewton - BFGS quasi-newton optimizer¶

 `quasinewton` Run a quasinewton optimization on the problem.

BFGS quasi-newton optimizer.

All modules in this file are implemented from the book “Numerical Methods for Unconstrained Optimization and Nonlinear Equations” by J.E. Dennis and Robert B. Schnabel (Only a few minor modifications are done).

The interface is through the `quasinewton()` function. Here is an example call:

```n = 2
x0 = [-0.9 0.9]'
fn = lambda p: (1-p)**2 + 100*(p-p**2)**2
grad = lambda p: array([-2*(1-p) - 400*(p-p**2)*p, 200*p])
Sx = ones(n,1)
typf = 1                       # todo. see what default value is the best
macheps = eps
eta = eps
maxstep = 100
steptol = 1e-12                # do not let steptol larger than 1e-9
itnlimit = 1000
result = quasinewton(fn, x0, grad, Sx, typf,
macheps, eta, maxstep, gradtola, steptol, itnlimit)
print("status code %d"%result['status'])
print("x_min=%s, f(x_min)=%g"%(str(result['x']),result['fx']))
print("iterations, function calls, linesearch function calls",
result['iterations'],result['evals'],result['linesearch_evals'])
```
bumps.quasinewton.quasinewton(fn, x0=None, grad=None, Sx=None, typf=1, macheps=None, eta=None, maxstep=100, gradtol=1e-06, steptol=1e-12, itnlimit=2000, abort_test=None, monitor=<function <lambda>>)[source]

Run a quasinewton optimization on the problem.

fn(x) is the cost function, which takes a point x and returns a scalar fx.

x0 is the initial point

Sx is a scale vector indicating the typical values for parameters in the fitted result. This is used for a variety of things such as setting the step size in the finite difference approximation to the gradient, and controlling numerical accuracy in calculating the Hessian matrix. If for example some of your model parameters are in the order of 1e-6, then Sx for those parameters should be set to 1e-6. Default: [1, …]

typf is the typical value for f(x) near the minimum. This is used along with gradtol to check the gradient stopping condition. Default: 1

macheps is the minimum value that can be added to 1 to produce a number not equal to 1. Default: numpy.finfo(float).eps

maxstep is the maximum step size in any gradient step, after normalizing by Sx. Default: 100

gradtol is a stopping condition for the fit based on the amount of improvement expected at the next step. Default: 1e-6

steptol is a stopping condition for the fit based on the size of the step. Default: 1e-12

itnlimit is the maximum number of steps to take before stopping. Default: 2000

abort_test is a function which tests whether the user has requested abort. Default: None.

monitor(x,fx,step) is called every iteration so that a user interface function can monitor the progress of the fit. Default: lambda **kw: True

Returns the fit result as a dictionary:

status is a status code indicating why the fit terminated. Turn the status code into a string with STATUS[result.status]. Status values vary from 1 to 9, with 1 and 2 indicating convergence and the remaining codes indicating some form of premature termination.

x is the minimum point

fx is the value fn(x) at the minimum

H is the approximate Hessian matrix, which is the inverse of the covariance matrix

L is the cholesky decomposition of H+D, where D is a small correction to force H+D to be positive definite. To compute parameter uncertainty

iterations is the number of iterations

evals is the number of function evaluations

linesearch_evals is the number of function evaluations for line search