bfgs: Optimization using BFGS

In rHanso: An R Implementation of Hybrid Algorithm for Non-Smooth Optimization (HANSO)

Description

BFGS is normally used for optimizing smooth, not necessarily convex, functions, for which the convergence rate is generically superlinear. But it also works very well for functions that are nonsmooth at their minimizers, typically with a linear convergence rate and a final inverse Hessian approximation that is very ill conditioned, as long as a weak Wolfe line search is used. This version of BFGS will work well both for smooth and nonsmooth functions and has a stopping criterion that applies for both cases, described above.

Usage

bfgs(fn, gr, nvar=0, nstart = 10, x0 = NULL, upper = 1,
      lower = 0,  maxit = 1000, normtol = 1e-06,
      fvalquit = -Inf, xnormquit = Inf, nvec = 0,
      prtlevel = 1, strongwolfe = 0, wolfe1 = 1e-04,
      wolfe2 = 0.5, quitLSfail = 1, ngrad = 0, evaldist = 1e-04,
      H0 = NULL, scale = 1)

Arguments

fn	A function to be minimized. fn(x) takes input as a vector of parameters over which minimization is to take place. fn() returns a scaler.
gr	A function to return the gradient for fn(x).
nvar	Number of parameters that fn() takes.
nstart	Number of initial guesses. Default is 10.
x0	Matrix, with dimension (nvar x nstart), each column represent an initial guess.
upper	upper bound for the initial value
lower	lower bound for the initial value
maxit	maximum number of iterations.
normtol	termination tolerance on d: smallest vector in convex hull of up to ngrad gradients (default: 1e-6)
fvalquit	quit if f drops below this value.
xnormquit	quit if norm(x) drops below this.
nvec	0 for saving bfgs values.
prtlevel	prints messages if this is 1
strongwolfe	if this is 1, strong line search is used, otherwise, weak line search is used. Default is 0.
wolfe1	wolfe line search parameter 1.
wolfe2	wolfe line search parameter 2.
quitLSfail	1 if want to quit when line search fails. 0 otherwise.
ngrad	number of gradients willing to save and use in solving QP to check optimality tolerance on smallest vector in their convex hull; see also next two options
evaldist	the gradients used in the termination test qualify only if they are evaluated at points approximately within distance options.evaldist of x
H0	Initial inverse hessian approximation. Default is NULL
scale	1 to scale H0 at first iteration, 0 otherwise.

Value

Returns a list containing the following fields:

x	a matrix with k'th column containing final value of x obtained from k'th column of x0.
f	a vector of final obtained minimum values of fn() at the initial points.
d	the final smallest vector in the convex hull of the saved gradient.
H	final BFGS inverse hessian approximation
iter	number of iterations
message	any warnings / messages for checking the termination condition.
X	iterates, where saved gradients were evaluated.
G	saved gradients used for computation.
w	weights giving the smallest vector.

Author(s)

Copyright (c) 2010 Michael Overton for Matlab code and documentation, with permission converted to R by Abhirup Mallik (and Hans W Borchers).

References

A.S. Lewis and M.L. Overton, Nonsmooth Optimization via BFGS, 2008.

See Also

hanso, gradsamp, nlcg, shor

Examples

fr <- function(x) {   ## Rosenbrock Banana function
x1 <- x[1]
x2 <- x[2]
100 * (x2 - x1 * x1)^2 + (1 - x1)^2
}
grr <- function(x) { ## Gradient of 'fr'
x1 <- x[1]
x2 <- x[2]
c(-400 * x1 * (x2 - x1 * x1) - 2 * (1 - x1),
200 *      (x2 - x1 * x1))
}
res=bfgs(fr,grr,nvar=2) # using all the default parameters, using weak wolfe search
res
res=bfgs(fr,grr,nvar=2,strongwolfe=1) # using strong wolfe search
res
# Nesterov's function in 5 dimention
# example not run
#fnNesterov1 <- function(x) {
#  n <- length(x)
#  x2 <- x[2:n]; x1 <- x[1:(n-1)]
#  1/4*(x[1]-1)^2 + sum(abs(x2-2*x1^2+1))
#}

#res=bfgs(fnNesterov1,nvar=5)
#res

rHanso documentation built on May 2, 2019, 5:26 p.m.