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Structuring a Neural Lyapunov function

Users have two ways to specify the structure of their neural Lyapunov function: through Lux and through NeuralLyapunov. NeuralLyapunov.jl is intended for use with NeuralPDE.jl, which is itself intended for use with Lux.jl. Users therefore must provide a Lux model representing the neural network $\phi(x)$ which will be trained, regardless of the transformation they use to go from $\phi$ to $V$.

In some cases, users will find it simplest to make $\phi(x)$ a simple multilayer perceptron and specify their neural Lyapunov function structure as a transformation of the function $\phi$ to the function $V$ using the NeuralLyapunovStructure struct, detailed below.

In other cases (particularly when they find the NeuralPDE parser has trouble tracing their structure), users may wish to represent their neuralLyapunov function structure using Lux.jl/Boltz.jl layers, integrating them into $\phi(x)$ and letting $V(x) = \phi(x)$ (as in NoAdditionalStructure, detailed below).

Users may also combine the two methods, particularly if they find that their structure can be broken down into a component that the NeuralPDE parser has trouble tracing but exists in Lux/Boltz, and another aspect that can be written easily using a NeuralLyapunovStructure but does not correspond any existing Lux/Boltz layer. (Such an example will be provided below.)

NeuralLyapunov.jl supplies two Lux structures and two pooling layers for structuring $\phi(x)$, along with three NeuralLyapunovStructure transformations. Additionally, users can always specify a custom structure using the NeuralLyapunovStructure struct.

Pre-defined NeuralLyapunov transformations

The simplest structure is to train the neural network directly to be the Lyapunov function, which can be accomplished using an NoAdditionalStructure. This is particularly useful with the pre-defined Lux structures detailed in the following section.

NeuralLyapunov.NoAdditionalStructure — Function
NoAdditionalStructure()

Create a NeuralLyapunovStructure where the Lyapunov function is the neural network evaluated at the state. This does impose any additional structure to enforce any Lyapunov conditions.

Corresponds to $V(x) = ϕ(x)$, where $ϕ$ is the neural network.

Dynamics are assumed to be in f(state, p, t) form, as in an ODEFunction. For f(state, input, p, t), consider using add_policy_search.

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The condition that the Lyapunov function $V(x)$ must be minimized uniquely at the fixed point $x_0$ is often represented as a requirement for $V(x)$ to be positive away from the fixed point and zero at the fixed point. Put mathematically, it is sufficient to require $V(x) > 0 \, \forall x \ne x_0$ and $V(x_0) = 0$. We call such functions positive definite (around the fixed point $x_0$).

Two structures are provided which partially or fully enforce the minimization condition: NonnegativeStructure, which structurally enforces $V(x) \ge 0$ everywhere, and PositiveSemiDefiniteStructure, which additionally enforces $V(x_0) = 0$.

NeuralLyapunov.NonnegativeStructure — Function
NonnegativeStructure(network_dim; <keyword_arguments>)

Create a NeuralLyapunovStructure where the Lyapunov function is the L2 norm of the neural network output plus a constant δ times a function pos_def.

Corresponds to $V(x) = \lVert ϕ(x) \rVert^2 + δ \, \texttt{pos\_def}(x, x_0)$, where $ϕ$ is the neural network and $x_0$ is the equilibrium point.

This structure ensures $V(x) ≥ 0 \, ∀ x$ when $δ ≥ 0$ and pos_def is always nonnegative. Further, if $δ > 0$ and pos_def is strictly positive definite around fixed_point, the structure ensures that $V(x)$ is strictly positive away from fixed_point. In such cases, the minimization condition reduces to ensuring $V(x_0) = 0$, and so DontCheckNonnegativity(true) should be used.

Arguments

  • network_dim: output dimensionality of the neural network.

Keyword Arguments

  • δ: weight of pos_def, as above; defaults to 0.
  • pos_def(state, fixed_point): a function that is positive (semi-)definite in state around fixed_point; defaults to $\log(1 + \lVert x - x_0 \rVert^2)$.
  • grad_pos_def(state, fixed_point): the gradient of pos_def with respect to state at state. If isnothing(grad_pos_def) (as is the default), the gradient of pos_def will be evaluated using grad.
  • grad: a function for evaluating gradients to be used when isnothing(grad_pos_def); defaults to, and expects the same arguments as, ForwardDiff.gradient.

Dynamics are assumed to be in f(state, p, t) form, as in an ODEFunction. For f(state, input, p, t), consider using add_policy_search.

See also: DontCheckNonnegativity

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NeuralLyapunov.PositiveSemiDefiniteStructure — Function
PositiveSemiDefiniteStructure(network_dim; <keyword_arguments>)

Create a NeuralLyapunovStructure where the Lyapunov function is the product of a positive (semi-)definite function pos_def which does not depend on the network and a nonnegative function non_neg which does depend the network.

Corresponds to $V(x) = \texttt{pos\_def}(x, x_0) * \texttt{non\_neg}(ϕ, x, x_0)$, where $ϕ$ is the neural network and $x_0$ is the equilibrium point.

This structure ensures $V(x) ≥ 0$. Further, if pos_def is strictly positive definite fixed_point and non_neg is strictly positive (as is the case for the default values of pos_def and non_neg), then this structure ensures $V(x)$ is strictly positive definite around fixed_point. In such cases, the minimization condition is satisfied structurally, so DontCheckNonnegativity(false) should be used.

Arguments

  • network_dim: output dimensionality of the neural network.

Keyword Arguments

  • pos_def(state, fixed_point): a function that is positive (semi-)definite in state around fixed_point; defaults to $\log(1 + \lVert x - x_0 \rVert^2)$.
  • non_neg(net, state, fixed_point): a nonnegative function of the neural network; note that net is the neural network $ϕ$, and net(state) is the value of the neural network at a point $ϕ(x)$; defaults to $1 + \lVert ϕ(x) \rVert^2$.
  • grad_pos_def(state, fixed_point): the gradient of pos_def with respect to state at state. If isnothing(grad_pos_def) (as is the default), the gradient of pos_def will be evaluated using grad.
  • grad_non_neg(net, J_net, state, fixed_point): the gradient of non_neg with respect to state at state; J_net is a function outputting the Jacobian of net at the input. If isnothing(grad_non_neg) (as is the default), the gradient of non_neg will be evaluated using grad.
  • grad: a function for evaluating gradients to be used when isnothing(grad_pos_def) || isnothing(grad_non_neg); defaults to, and expects the same arguments as, ForwardDiff.gradient.

Dynamics are assumed to be in f(state, p, t) form, as in an ODEFunction. For f(state, input, p, t), consider using add_policy_search.

See also: DontCheckNonnegativity

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Pre-defined Lux structures

Regardless of what NeuralLyapunov transformation is used to transform $\phi$ into $V$, users should carefully consider their choice of $\phi$. Two options provided by NeuralLyapunov, intended to be used with NoAdditionalStructure, are AdditiveLyapunovNet and MultiplicativeLyapunovNet. These each wrap a different Lux model, effectively performing the transformation from $\phi$ to $V$ within the Lux ecosystem, rather than in the NeuralPDE/ModelingToolkit symbolic ecosystem.

AdditiveLyapunovNet is based on (Gaby et al., 2021), and MultiplicativeLyapunovNet is an analogous structure combining the neural term and the positive definite term via multiplication instead of addition.

NeuralLyapunov.AdditiveLyapunovNet — Function
AdditiveLyapunovNet(ϕ; ψ, m, r, dim_ϕ, dim_m, fixed_point)

Construct a Lyapunov-Net with the following structure:

\[ V(x) = ψ(ϕ(x) - ϕ(x_0)) + r(m(x) - m(x_0)),\]

where $x_0$ is fixed_point and the functions are defined as below. If the functions meet the conditions listed below, the resulting model will be positive definite (around fixed_point), as the $r$ term will be positive definite and the $ψ$ term will be positive semidefinite.

Arguments

  • ϕ: The base neural network model; its output dimension should be dim_ϕ.
  • ψ: A Lux layer representing a positive semidefinite function that maps the output of ϕ to a scalar value; defaults to SoSPooling() (i.e., $\lVert ⋅ \rVert^2$). Users may provide a function instead of a Lux layer, in which case it will be wrapped into a layer via Lux.WrappedFunction.
  • m: Optional pre-processing layer for use before r. This layer should output a vector of dimension dim_m and $m(x) = m(x_0)$ should imply that $x$ is an equilibrium to be analyzed by the Lyapunov function. Defaults to Lux.NoOpLayer(), which is typically the right choice when analyzing a single equilibrium point. Consider using a Boltz.Layers.PeriodicEmbedding if any of the state variables are periodic. Users may provide a function instead of a Lux layer, in which case it will be wrapped into a layer via Lux.WrappedFunction.
  • r: A Lux layer representing a positive definite function that maps the output of m to a scalar value; defaults to SoSPooling() (i.e., $\lVert ⋅ \rVert^2$). Users may provide a function instead of a Lux layer, in which case it will be wrapped into a layer via Lux.WrappedFunction.
  • dim_ϕ: The dimension of the output of ϕ.
  • dim_m: The dimension of the output of m; defaults to length(fixed_point) when fixed_point is provided and dim_m isn't. Users must provide at least one of dim_m and fixed_point.
  • fixed_point: A vector of length dim_m representing the fixed point; defaults to zeros(dim_m) when dim_m is provided and fixed_point isn't. Users must provide at least one of dim_m and fixed_point.
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NeuralLyapunov.MultiplicativeLyapunovNet — Function
MultiplicativeLyapunovNet(ϕ; ζ, m, r, dim_m, fixed_point)

Construct a Lyapunov-Net with the following structure:

\[ V(x) = ζ(ϕ(x)) (r(m(x) - m(x_0))),\]

where $x_0$ is fixed_point and the functions are defined as below. If the functions meet the conditions listed below, the resulting model will be positive definite (around fixed_point), as the $r$ term will be positive definite and the $ζ$ term will be strictly positive.

Arguments

  • ϕ: The base neural network model.
  • ζ: A Lux layer representing a strictly positive function that maps the output of ϕ to a scalar value; defaults to StrictlyPositiveSoSPooling() (i.e., $1 + \lVert ⋅ \rVert^2$). Users may provide a function instead of a Lux layer, in which case it will be wrapped into a layer via Lux.WrappedFunction.
  • m: Optional pre-processing layer for use before r. This layer should output a vector of dimension dim_m and $m(x) = m(x_0)$ should imply that $x$ is an equilibrium to be analyzed by the Lyapunov function. Defaults to Lux.NoOpLayer(), which is typically the right choice when analyzing a single equilibrium point. Consider using a Boltz.Layers.PeriodicEmbedding if any of the state variables are periodic. Users may provide a function instead of a Lux layer, in which case it will be wrapped into a layer via Lux.WrappedFunction.
  • r: A Lux layer representing a positive definite function that maps the output of m to a scalar value; defaults to SoSPooling() (i.e., $\lVert ⋅ \rVert^2$). Users may provide a function instead of a Lux layer, in which case it will be wrapped into a layer via Lux.WrappedFunction.
  • dim_m: The dimension of the output of m; defaults to length(fixed_point) when fixed_point is provided and dim_m isn't.
  • fixed_point: A vector of length dim_m representing the fixed point; defaults to zeros(dim_m) when dim_m is provided and fixed_point isn't.
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Note that using NoAdditionalStructure with MultiplicativeLyapunovNet wrapping a Lux model $\phi$ is the same as using PositiveSemiDefiniteStructure the same $\phi$, but in the former the transformation is handled in the Lux ecosystem and in the latter the transformation is handled in the NeuralPDE/ModelingToolkit ecosystem. Similarly, using NonnegativeStructure with Boltz.Layers.ShiftTo is analogous to using NoAdditionalStructure with AdditiveLyapunovNet. Because the NeuralPDE parser cannot process $\phi$ being evaluated at two different points (in this case $x$ and $x_0$), we cannot represent this structure purely in the NeuralPDE/ModelingToolkit ecosystem.

Helper layers provided for the above structures are also exported:

Defining your own neural Lyapunov function structure with NeuralLyapunovStructure

To define a new structure for a neural Lyapunov function, one must specify the form of the Lyapunov candidate $V$ and its time derivative along a trajectory $\dot{V}$, as well as how to call the dynamics $f$. Additionally, the dimensionality of the output of the neural network must be known in advance.

NeuralLyapunov.NeuralLyapunovStructure — Type
NeuralLyapunovStructure(V, V̇, f_call, network_dim)

Specifies the structure of the neural Lyapunov function and its derivative.

Allows the user to define the Lyapunov in terms of the neural network, potentially structurally enforcing some Lyapunov conditions.

Fields

  • V(phi, state, fixed_point): outputs the value of the Lyapunov function at state.
  • V̇(phi, J_phi, dynamics, state, params, t, fixed_point): outputs the time derivative of the Lyapunov function at state.
  • f_call(dynamics, phi, state, params, t): outputs the derivative of the state; this is useful for making closed-loop dynamics which depend on the neural network, such as in the policy search case.
  • network_dim: the dimension of the output of the neural network.

phi and J_phi above are both functions of state alone.

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Calling the dynamics

Very generally, the dynamical system can be a system of ODEs $\dot{x} = f(x, u, p, t)$, where $u$ is a control input, $p$ contains parameters, and $f$ depends on the neural network in some way. To capture this variety, users must supply the function f_call(dynamics, phi, state, p, t).

The most common example is when dynamics takes the same form as an ODEFunction. i.e., $\dot{x} = \texttt{dynamics}(x, p, t)$. In that case, f_call(dynamics, phi, state, p, t) = dynamics(state, p, t).

Suppose instead, the dynamics were supplied as a function of state alone: $\dot{x} = \texttt{dynamics}(x)$. Then, f_call(dynamics, phi, state, p, t) = dynamics(state).

Finally, suppose $\dot{x} = \texttt{dynamics}(x, u, p, t)$ has a unidimensional control input that is being trained (via policy search) to be the second output of the neural network. Then f_call(dynamics, phi, state, p, t) = dynamics(state, phi(state)[2], p, t).

Note that, despite the inclusion of the time variable $t$, NeuralLyapunov.jl currently only supports time-invariant systems, so only t = 0.0 is used.

Structuring $V$ and $\dot{V}$

The Lyapunov candidate function $V$ gets specified as a function V(phi, state, fixed_point), where phi is the neural network as a function phi(state). Note that this form allows $V(x)$ to depend on the neural network evaluated at points other than just the input $x$.

The time derivative $\dot{V}$ is similarly defined by a function V̇(phi, J_phi, dynamics, state, params, t, fixed_point). The function J_phi(state) gives the Jacobian of the neural network phi at state. The function dynamics is as above (with parameters params).

References