conjugate gradient descent · 2017-09-25 · 3 motivation conjugate direction methods can be...
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Conjugate Gradient Descent
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Conjugate Direction Methods
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Motivation Conjugate direction methods can be regarded as being between the method of steepest descent (first-order method that uses gradient) and Newton’s method (second-order method that uses Hessian as well).
Motivation:
! steepest descent is slow. Goal: Accelerate it!
! Newton method is fast… BUT: we need to calculate the inverse of the Hessian
matrix…
Something between steepest descent and Newton method?
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Conjugate Direction Methods Goal: • Accelerate the convergence rate of steepest descent • while avoiding the high computational cost of Newton’s
method
Originally developed for solving the quadratic problem:
Equivalently, our goal is to solve:
Conjugate direction methods can solve this problem at most n iterations (usually for large n less is enough)
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Conjugate Direction Methods
! algorithm for the numerical solution of linear equations, whose matrix Q is symmetric and positive-definite.
! An iterative method, so it can be applied to systems that are too large to be handled by direct methods (such as the Cholesky decomposition.)
! Algorithm for seeking minima of nonlinear equations.
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Numerical Experiments
A comparison of
* gradient descent with optimal step size (in green) and
* conjugate vector (in red)
for minimizing a quadratic function.
Conjugate gradient converges in at most n steps (here n=2).
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Conjugate directions
! In the applications that we consider, the matrix Q will be positive definite but this is not inherent in the basic definition.
! If Q = 0, any two vectors are conjugate.
! if Q = I, conjugacy is equivalent to the usual notion of orthogonality.
Definition [Q-conjugate directions]
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Linear independence lemma
Lemma [Linear Independence]
Proof: [Proof by contradiction]
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Why is Q-conjugacy useful?
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Quadratic problem
Goal:
the unique solution to this problem is also the unique solution to the linear equation:
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Importance of Q-conjugancy
Therefore,
Standard orthogonality is not enough anymore,
We need to use Q-conjugacy.
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Importance of Q-conjugancy
No need to do matrix inversion! We only need to calculate inner products.
This can be generalized further such a way that the starting point of the iteration can be arbitrary x0
n - 1 n - 1
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Conjugate Direction Theorem
Theorem [Conjugate Direction Theorem]
In the previous slide we had
No need to do matrix inversion! We only need to calculate inner products.
[update rule]
[gradient of f]
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Proof
Therefore, it is enough to prove that with these αk values we have
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Proof
Therefore,
We already know
Q.E.D.
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Another motivation for Q-conjugacy
Goal:
Therefore,
n separate 1-dimensional optimization problems!
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Expanding Subspace Theorem
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Expanding Subspace Theorem
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Expanding Subspace Theorem Theorem [Expanding Subspace Theorem]
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Expanding Subspace Theorem Proof
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Proof
By definition,
Therefore,
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Proof
We have proved
By definition,
Therefore,
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Proof
Since
[By induction assumption],
Therefore,
Q.E.D.
[We have proved this]
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Corollary of Exp. Subs. Theorem Corollary
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THE CONJUGATE GRADIENT METHOD
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THE CONJUGATE GRADIENT METHOD
! The conjugate gradient method is a conjugate direction method
! Selects the successive direction vectors as a conjugate version of the successive gradients obtained as the method progresses.
! The conjugate directions are not specified beforehand, but rather are determined sequentially at each step of the iteration.
Given d0,…, dn-1, we already have an update rule for αk
How should we choose vectors d0,…, dn-1?
The conjugate gradient method
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THE CONJUGATE GRADIENT METHOD
Advantages
! Simple update rule
! the directions are based on the gradients, therefore the process makes good uniform progress toward the solution at every step.
For arbitrary sequences of conjugate directions the progress may be slight until the final few steps
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Conjugate Gradient Algorithm
! The CGA is only slightly more complicated to implement than the method of steepest descent but converges in a finite number of steps on quadratic problems.
! In contrast to Newton method, there is no need for matrix inversion.
Conjugate Gradient Algorithm
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Conjugate Gradient Theorem
To verify that the algorithm is a conjugate direction algorithm, all we need is to verify that the vectors d0,…,dk are Q-orthogonal.
Theorem [Conjugate Gradient Theorem]
The conjugate gradient algorithm is a conjugate direction method.
a)
b)
c)
d)
e)
Only αk needs matrix Q in the algorithm!
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Proofs
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EXTENSION TO NONQUADRATIC PROBLEMS
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EXTENSION TO NONQUADRATIC PROBLEMS
Do quadratic approximation
This is similar to Newton’s method. [f is approximated by a quadratic function]
! When applied to nonquadratic problems, conjugate gradient methods will not usually terminate within n steps.
! After n steps, we can restart the process from this point and run the algorithm for another n steps…
Goal:
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Conjugate Gradient Algorithm for nonquadratic functions
Step 1
Step 2
Step 3
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Properties of CGA
! An attractive feature of the algorithm is that, just as in the pure form of Newton’s method, no line searching is required at any stage.
! The algorithm converges in a finite number of steps for a quadratic problem.
! The undesirable features are that Hessian matrix must be evaluated at each point.
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LINE SEARCH METHODS
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Step 1
Step 2
Step 3
Fletcher–Reeves method
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Line search method
Hessian is not used in the algorithm
In the quadratic case it is identical to the original conjugate direction algorithm
Fletcher–Reeves method
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Polak–Ribiere method
Again this leads to a value identical to the standard formula in the quadratic case.
Experimental evidence seems to favor the Polak–Ribiere method over other methods of this general type.
Same as Fletcher–Reeves method, BUT:
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Convergence rate Under some conditions the line search method is globally convergent.
Under some conditions, the rate is
[since one complete n step cycle solves a quadratic problem similarly
To the Newton method]
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Acceleration
Conjugate gradient method attempts to accelerate gradient descent by building in momentum.
Recall:
First one implies:
Substituting last two into first one:
dk�1 =xk � xk�1
↵k�1
= xk � ↵kgk +↵k�k�1
↵k�1(xk � xk�1)
xk+1 = xk � ↵kgk + ↵k�k�1dk�1
Momentum term
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Summary
! Conjugate Direction Methods
- conjugate directions
! Minimizing quadratic functions
! Conjugate Gradient Methods for nonquadratic functions
- Line search methods
* Fletcher–Reeves
* Polak–Ribiere