In the unconstrained minimization problem, the Wolfe conditions are a set of inequalities for performing inexact line search, especially in quasi-Newton methods, first published by Philip Wolfe in 1969.
In these methods the idea is to find
for some smooth . Each step often involves approximately solving the subproblem
where is the current best guess, is a search direction, and is the step length.
The inexact line searches provide an efficient way of computing an acceptable step length that reduces the objective function 'sufficiently', rather than minimizing the objective function over exactly. A line search algorithm can use Wolfe conditions as a requirement for any guessed , before finding a new search direction .
Armijo rule and curvature
Denote a univariate function restricted to the direction as . A step length is said to satisfy the Wolfe conditions if the following two inequalities hold:
- i) ,
- ii) ,
with . (In examining condition (ii), recall that to ensure that is a descent direction, we have .)
is usually chosen to be quite small while is much larger; Nocedal gives example values of and for Newton or quasi-Newton methods and for the nonlinear conjugate gradient method. Inequality i) is known as the Armijo rule and ii) as the curvature condition; i) ensures that the step length decreases 'sufficiently', and ii) ensures that the slope has been reduced sufficiently.
Strong Wolfe condition on curvature
The Wolfe conditions, however, can result in a value for the step length that is not close to a minimizer of . If we modify the curvature condition to the following,
then i) and iia) together form the so-called strong Wolfe conditions, and force to lie close to a critical point of .
The principal reason for imposing the Wolfe conditions in an optimization algorithm where is to ensure convergence of the gradient to zero. In particular, if the cosine of the angle between and the gradient,
is bounded away from zero and the i) and ii) conditions hold, then .
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- Armijo, Larry (1966). "Minimization of functions having Lipschitz continuous first partial derivatives". Pacific J. Math. 16 (1): 1–3.