Gradient Descent

gradient_descent(M, f, grad_f, p; kwargs...)

perform a gradient descent

\[p_{k+1} = \operatorname{retr}_{p_k}\bigl( s_k\operatorname{grad}f(p_k) \bigr), \qquad k=0,1,…\]

with different choices of the stepsize $s_k$ available (see stepsize option below).

Input

• M – a manifold $\mathcal M$
• f – a cost function $f: \mathcal M→ℝ$ to find a minimizer $p^*$ for
• grad_f – the gradient $\operatorname{grad}f: \mathcal M → T\mathcal M$ of f
  • as a function (M, p) -> X or a function (M, X, p) -> X
• p – an initial value p $= p_0 ∈ \mathcal M$

Optional

• direction – IdentityUpdateRule perform a processing of the direction, e.g.
• evaluation – (AllocatingEvaluation) specify whether the gradient works by allocation (default) form grad_f(M, p) or InplaceEvaluation in place, i.e. is of the form grad_f!(M, X, p).
• retraction_method – (default_retraction_method(M, typeof(p))) a retraction to use
• stepsize – (ConstantStepsize(1.)) specify a Stepsize functor.
• stopping_criterion – (StopWhenAny(StopAfterIteration(200),StopWhenGradientNormLess(10.0^-8))) a functor inheriting from StoppingCriterion indicating when to stop.

All other keyword arguments are passed to decorate_state! for decorators.

Output

the obtained (approximate) minimizer $p^*$. To obtain the whole final state of the solver, see get_solver_return for details

gradient_descent!(M, f, grad_f, p)

perform a gradient_descent

\[p_{k+1} = \operatorname{retr}_{p_k}\bigl( s_k\operatorname{grad}f(p_k) \bigr)\]

in place of p with different choices of $s_k$ available.

Input

• M – a manifold $\mathcal M$
• f – a cost function $F:\mathcal M→ℝ$ to minimize
• grad_f – the gradient $\operatorname{grad}F:\mathcal M→ T\mathcal M$ of F
• p – an initial value $p ∈ \mathcal M$

For more options, especially Stepsizes for $s_k$, see gradient_descent

GradientDescentState{P,T} <: AbstractGradientSolverState

Describes a Gradient based descent algorithm, with

Fields

a default value is given in brackets if a parameter can be left out in initialization.

• p – (rand(M)` the current iterate
• X – (zero_vector(M,p)) the current gradient $\operatorname{grad}f(p)$, initialised to zero vector.
• stopping_criterion – (StopAfterIteration(100)) a StoppingCriterion
• stepsize – (default_stepsize(M, GradientDescentState)) a Stepsize
• direction - (IdentityUpdateRule) a processor to compute the gradient
• retraction_method – (default_retraction_method(M, typeof(p))) the retraction to use, defaults to the default set for your manifold.

Constructor

GradientDescentState(M, p=rand(M); X=zero_vector(M, p), kwargs...)

Generate gradient descent options, where X can be used to set the tangent vector to store the gradient in a certain type; it will be initialised accordingly at a later stage. All following fields are keyword arguments.

See also

gradient_descent

DirectionUpdateRule

A general functor, that handles direction update rules. It's field(s) is usually only a StoreStateAction by default initialized to the fields required for the specific coefficient, but can also be replaced by a (common, global) individual one that provides these values.

IdentityUpdateRule <: DirectionUpdateRule

The default gradient direction update is the identity, i.e. it just evaluates the gradient.

MomentumGradient <: DirectionUpdateRule

Append a momentum to a gradient processor, where the last direction and last iterate are stored and the new is composed as $η_i = m*η_{i-1}' - s d_i$, where $sd_i$ is the current (inner) direction and $η_{i-1}'$ is the vector transported last direction multiplied by momentum $m$.

Fields

• p_old - (rand(M)) remember the last iterate for parallel transporting the last direction
• momentum – (0.2) factor for momentum
• direction – internal DirectionUpdateRule to determine directions to add the momentum to.
• vector_transport_method – default_vector_transport_method(M, typeof(p)) vector transport method to use
• X_old – (zero_vector(M,x0)) the last gradient/direction update added as momentum

Constructors

Add momentum to a gradient problem, where by default just a gradient evaluation is used

MomentumGradient(
    M::AbstractManifold;
    p=rand(M),
    s::DirectionUpdateRule=IdentityUpdateRule();
    X=zero_vector(p.M, x0), momentum=0.2
    vector_transport_method=default_vector_transport_method(M, typeof(p)),
)

Initialize a momentum gradient rule to s. Note that the keyword agruments p and X will be overriden often, so their initialisation is meant to set the to certain types of points or tangent vectors, if you do not use the default types with respect to M.

AverageGradient <: DirectionUpdateRule

Add an average of gradients to a gradient processor. A set of previous directions (from the inner processor) and the last iterate are stored, average is taken after vector transporting them to the current iterates tangent space.

Fields

• gradients – (fill(zero_vector(M,x0),n)) the last n gradient/direction updates
• last_iterate – last iterate (needed to transport the gradients)
• direction – internal DirectionUpdateRule to determine directions to apply the averaging to
• vector_transport_method - vector transport method to use

Constructors

AverageGradient(
    M::AbstractManifold,
    p::P=rand(M);
    n::Int=10
    s::DirectionUpdateRule=IdentityUpdateRule();
    gradients = fill(zero_vector(p.M, o.x),n),
    last_iterate = deepcopy(x0),
    vector_transport_method = default_vector_transport_method(M, typeof(p))
)

Add average to a gradient problem, where

• n determines the size of averaging
• s is the internal DirectionUpdateRule to determine the gradients to store
• gradients can be prefilled with some history
• last_iterate stores the last iterate
• vector_transport_method determines how to transport all gradients to the current iterates tangent space before averaging

Nesterov <: DirectionUpdateRule

Fields

• γ
• μ the strong convexity coefficient
• v (=$=v_k$, $v_0=x_0$) an interims point to compute the next gradient evaluation point y_k
• shrinkage (= i -> 0.8) a function to compute the shrinkage $β_k$ per iterate.

Let's assume $f$ is $L$-Lipschitz and $μ$-strongly convex. Given

• a step size $h_k<\frac{1}{L}$ (from the GradientDescentState
• a shrinkage parameter $β_k$
• and a current iterate $x_k$
• as well as the interims values $γ_k$ and $v_k$ from the previous iterate.

This compute a Nesterov type update using the following steps, see ^{[ZhangSra2018]}

1. Copute the positive root, i.e. $α_k∈(0,1)$ of $α^2 = h_k\bigl((1-α_k)γ_k+α_k μ\bigr)$.
2. Set $\bar γ_k+1 = (1-α_k)γ_k + α_kμ$
3. $y_k = \operatorname{retr}_{x_k}\Bigl(\frac{α_kγ_k}{γ_k + α_kμ}\operatorname{retr}^{-1}_{x_k}v_k \Bigr)$
4. $x_{k+1} = \operatorname{retr}_{y_k}(-h_k \operatorname{grad}f(y_k))$
5. $v_{k+1} = `\operatorname{retr}_{y_k}\Bigl(\frac{(1-α_k)γ_k}{\barγ_k}\operatorname{retr}_{y_k}^{-1}(v_k) - \frac{α_k}{\bar γ_{k+1}}\operatorname{grad}f(y_k) \Bigr)$
6. $γ_{k+1} = \frac{1}{1+β_k}\bar γ_{k+1}$

Then the direction from $x_k$ to $x_k+1$, i.e. $d = \operatorname{retr}^{-1}_{x_k}x_{k+1}$ is returned.

Constructor

Nesterov(M::AbstractManifold, p::P; γ=0.001, μ=0.9, schrinkage = k -> 0.8;
    inverse_retraction_method=LogarithmicInverseRetraction())

Initialize the Nesterov acceleration, where x0 initializes v.

DebugGradient <: DebugAction

debug for the gradient evaluated at the current iterate

Constructors

DebugGradient(; long=false, prefix= , format= "$prefix%s", io=stdout)

display the short (false) or long (true) default text for the gradient, or set the prefix manually. Alternatively the complete format can be set.

DebugGradientNorm <: DebugAction

debug for gradient evaluated at the current iterate.

Constructors

DebugGradientNorm([long=false,p=print])

display the short (false) or long (true) default text for the gradient norm.

DebugGradientNorm(prefix[, p=print])

display the a prefix in front of the gradientnorm.

DebugStepsize <: DebugAction

debug for the current step size.

Constructors

DebugStepsize(;long=false,prefix="step size:", format="$prefix%s", io=stdout)

display the a prefix in front of the step size.

RecordGradient <: RecordAction

record the gradient evaluated at the current iterate

Constructors

RecordGradient(ξ)

initialize the RecordAction to the corresponding type of the tangent vector.

RecordGradientNorm <: RecordAction

record the norm of the current gradient

RecordStepsize <: RecordAction

record the step size