example.md

Discovering Differential Equations with Neural ODEs

This example demonstrates how to use neural ODEs to discover underlying differential equations following the Universal Differential Equation [1] method.

The method is used for time-series data originating from ordinary differential equations (ODEs), in particular when not all of the terms of the ODE are known. For example, let $\mathbf{X}(t)\in {\mathbb{R}}^n$ be a time-series for $t\in [t_0 ,t_1 ]$ and suppose that we know $\mathbf{X}(t)$ satisfies an ODE of the form:

$$ \frac{\mathrm{d}\mathbf{X}}{\mathrm{d}t}(t)=\mathbf{f}(t,\mathbf{X}(t)),t\in [t_0 ,t_1 ],\mathbf{f}:[t_0 ,t_1 ]\times {\mathbb{R}}^n \to {\mathbb{R}}^n ,~~(1) $$

with initial condition $\mathbf{X}(t_0 )={\mathbf{X}}_0 .$

In the case that $\mathbf{f}$ is only partially known, for example $\mathbf{f}(t,\mathbf{X})=\mathbf{g}(t,\mathbf{X})+\mathbf{h}(t,\mathbf{X})$ where $\mathbf{g}:[t_0 ,t_1 ]\times {\mathbb{R}}^n \to {\mathbb{R}}^n$ is a known function, and $\mathbf{h}:[t_0 ,t_1 ]\times {\mathbb{R}}^n \to {\mathbb{R}}^n$ is unknown, the method proposes to model $\mathbf{h}$ by a universal approximator such as a neural network ${\mathbf{h}}_{\theta } :[t_0 ,t_1 ]\times {\mathbb{R}}^n \to {\mathbb{R}}^n$ with learnable parameters $\theta$.

When $\mathbf{h}$ is replaced by a neural network ${\mathbf{h}}_{\theta }$, equation (1) becomes a neural ODE which can be trained via gradient descent.

Once ${\mathbf{h}}{\theta }$ has been trained, symbolic regression methods can be used to find a functional representation of ${\mathbf{h}}{\theta }$, such as the SINDy method [2].

Training data

This example uses the Lotka-Volterra equations for demonstration purposes,

$$ \mathbf{X}(t)=(x(t),y(t)), $$

$$ \frac{\mathrm{d}x}{\mathrm{d}t}=x-\alpha xy, $$

$$ \frac{\mathrm{d}y}{\mathrm{d}t}=-y+\beta xy, $$

where $\alpha ,\beta$ are constants. For the purpose of the example suppose the terms $\alpha xy,\beta xy$ are unknown, and the linear terms are known. In the above notation that is $\mathbf{g}(t,(x,y))=(x,-y)$ are the known terms and $\mathbf{h}(t,(x,y)=(-\alpha xy,\beta xy)$ is unknown.

function dXdt = lotkaVolterraODE(X,nvp)
arguments
    X
    nvp.Alpha (1,1) double = 0.6
    nvp.Beta (1,1) double = 0.4
end
x = X(1);
y = X(2);
dxdt = x*(1 - nvp.Alpha*y);
dydt = y*(-1 + nvp.Beta *x);
dXdt = [dxdt; dydt];
end

Set $\alpha =0.6$, $\beta =0.4$, $t_0 =0$ and $\mathbf{X}(t_0 )={\mathbf{X}}_0 =(1,1)$.

alpha = 0.6;
beta = 0.4;
X0 = [1;1];

Solve the ODE using ode. For the purpose of the example, add random noise to the solution, as realistic is often noisy.

function X = noisySolve(F,ts,nvp)
arguments
    F (1,1) ode
    ts (1,:) double
    nvp.NoiseMagnitude (1,1) double = 2e-2
end
S = solve(F,ts);
X = S.Solution;
% Add noise to the solution, but not the initial condition.
noise = nvp.NoiseMagnitude * randn([size(X,1),size(X,2)-1]);
X(:,2:end) = X(:,2:end) + noise;
end

F = ode(...
    ODEFcn = @(t,X) lotkaVolterraODE(X, Alpha=alpha, Beta=beta),...
    InitialValue = X0);

ts = linspace(0,15,250);
X = noisySolve(F,ts);
scatter(ts,X(1,:),".");
hold on
scatter(ts,X(2,:),".");
title("Time-series data $\mathbf{X}(t) = (x(t),y(t))$", Interpreter="latex")
xlabel("$t$",Interpreter="latex")
legend(["$x(t)$","$y(t)$"], Interpreter="latex")
hold off

Take only the data $\mathbf{X}(t),t\in [0,3]$ as training data.

tsTrain = ts(ts<=3);
XTrain = X(:,ts<=3);

Design Universal ODE

Recall $\mathbf{f}(t,\mathbf{X})=\mathbf{g}(t,\mathbf{X})+\mathbf{h}(t,\mathbf{X})$ where $\mathbf{g}(t,\mathbf{X})=\mathbf{g}(t,(x,y))=(x,-y)$ is known, and $\mathbf{h}$ is to be approximated by a neural network ${\mathbf{h}}{\theta }$. Let ${\mathbf{f}}{\theta } (t,\mathbf{X})=\mathbf{g}(t,\mathbf{X})+{\mathbf{h}}{\theta } (t,\mathbf{X})$. By representing $\mathbf{g}$ as a neural network layer it is possible to represent ${\mathbf{f}}{\theta }$ as a neural network.

gFcn = @(X) [X(1,:); -X(2,:)];
gLayer = functionLayer(gFcn, Acceleratable=true, Name="g");

Define a neural network for ${\mathbf{h}}_{\theta }$.

activationLayer = functionLayer(@softplusActivation,Acceleratable=true);
depth = 3;
hiddenSize = 5;
stateSize = size(X,1);
hLayers = [
    featureInputLayer(stateSize,Name="X")
    repmat([fullyConnectedLayer(hiddenSize); activationLayer],[depth-1,1])
    fullyConnectedLayer(stateSize,Name="h")];
hNet = dlnetwork(hLayers);

Add the layer representing $\mathbf{g}$, connect the $\mathbf{X}$ input to $\mathbf{g}(\mathbf{X})$. Also add an additionLayer to perform the addition in ${\mathbf{f}}{\theta } =\mathbf{g}+{\mathbf{h}}{\theta }$ and connect ${\mathbf{h}}_{\theta }$ to this.

fNet = addLayers(hNet, [gLayer; additionLayer(2,Name="add")]);
fNet = connectLayers(fNet,"X","g");
fNet = connectLayers(fNet,"h","add/in2");

Analyse the network.

analyzeNetwork(fNet)

The dlnetwork specified by fNet represents the function ${\mathbf{f}}_{\theta } (t,\mathbf{X})$ in the neural ODE

$$ \frac{\mathrm{d}\mathbf{X}}{\mathrm{d}t}(t)={\mathbf{f}}_{\theta } (t,\mathbf{X}(t)). $$

To solve the neural ODE, place fNet inside a neuralODELayer, and solve for the times tsTrain.

neuralODE = [
    featureInputLayer(stateSize,Name="X0")
    neuralODELayer(fNet, tsTrain, GradientMode="adjoint")];
neuralODE = dlnetwork(neuralODE);

Train the neural ODE

Set the network to double precision using dlupdate.

neuralODE = dlupdate(@double, neuralODE);

Specify trainingOptions for ADAM and train. For a small neural ODE, often training on the CPU is faster than the GPU, as there is not sufficient parallelism in the neural ODE to make up for the overhead of sending data to the GPU.

opts = trainingOptions("adam",...
    Plots="training-progress",...
    MaxEpochs=600,...
    ExecutionEnvironment="cpu",...
    InputDataFormats="CB",...
    TargetDataFormats="CTB");
neuralODE = trainnet(XTrain(:,1),XTrain(:,2:end),neuralODE,"l2loss",opts);

    Iteration    Epoch    TimeElapsed    LearnRate    TrainingLoss
    _________    _____    ___________    _________    ____________
            1        1       00:00:00        0.001          2.2743
           50       50       00:00:03        0.001          1.3138
          100      100       00:00:05        0.001         0.97882
          150      150       00:00:07        0.001         0.73854
          200      200       00:00:09        0.001         0.56192
          250      250       00:00:12        0.001         0.43058
          300      300       00:00:14        0.001         0.33261
          350      350       00:00:16        0.001          0.2598
          400      400       00:00:18        0.001         0.20618
          450      450       00:00:21        0.001         0.16721
          500      500       00:00:23        0.001         0.13936
          550      550       00:00:25        0.001          0.1198
          600      600       00:00:27        0.001         0.10621
Training stopped: Max epochs completed

Next train with L-BFGS to optimize the training loss further.

opts = trainingOptions("lbfgs",...
    MaxIterations = 400,...
    Plots="training-progress",...
    ExecutionEnvironment="cpu",...
    GradientTolerance=1e-8,...
    StepTolerance=1e-8,...
    InputDataFormats="CB",...
    TargetDataFormats="CTB");
neuralODE = trainnet(XTrain(:,1),XTrain(:,2:end),neuralODE,"l2loss",opts);

    Iteration    TimeElapsed    TrainingLoss    GradientNorm    StepNorm
    _________    ___________    ____________    ____________    ________
            1       00:00:00         0.10012          1.5837    0.072727
           50       00:00:07        0.001433        0.024051     0.10165
          100       00:00:14      0.00083738        0.015113    0.071645
          150       00:00:23      0.00062907        0.013306    0.037026
          200       00:00:34      0.00058742      0.00016328  0.00036442
          250       00:01:12      0.00058725        0.000197   6.464e-05
Training stopped: Stopped manually

Identify equations for the universal approximator

Extract ${\mathbf{h}}_{\theta }$ from neuralODE.

fNetTrained = neuralODE.Layers(2).Network;
hNetTrained = removeLayers(fNetTrained,["g","add"]);
lrn = hNetTrained.Learnables;
lrn = dlupdate(@dlarray, lrn);
hNetTrained = initialize(hNetTrained);
hNetTrained.Learnables = lrn;

The SINDy method [2] takes a library of basis functions ${\mathbf{e}}_i (t,\mathbf{X}):[t_0 ,t_1 ]\times {\mathbb{R}}^n \to {\mathbb{R}}^n$ for $i=1,2,\ldots,N$ and sample points $(\tau_j ,{\mathbf{X}}_j )\in [t_0 ,t_1 ]\times {\mathbb{R}}^n$. The goal is to identify which terms ${\mathbf{e}}i$ represent $h{\theta }$.

Let $\mathbf{E}(t,\mathbf{X})=({\mathbf{e}}_1 (t,\mathbf{X}),\ldots,{\mathbf{e}}_N (t,\mathbf{X}))$ denote the matrix formed by concatenating all of the evaluations of the basis functions ${\mathbf{e}}i$. Identifying terms ${\mathbf{e}}i$ that make up ${\mathbf{h}}{\theta }$ can be written as the linear problem ${\mathbf{h}}{\theta } (t,\mathbf{X})=\mathbf{W}\mathbf{E}(t,\mathbf{X})$ for a matrix $\mathbf{W}$.

The SINDy method proposes to use a sparse regression method to solve for $\mathbf{W}$. The sequentially thresholded least squares method [2] iteratively solves for $\mathbf{W}$ in the above problem, and zeros out the terms in $\mathbf{W}$ with absolute value below a specified threshold.

Use the training data XTrain as the sample points ${\mathbf{X}}j$ and evaluate ${\mathbf{h}}{\theta } ({\mathbf{X}}_j )$.

Xextra = interp1(tsTrain,XTrain.', linspace(tsTrain(1), tsTrain(2), 100)).';
XSample = [XTrain,Xextra];
hEval = predict(hNetTrained,XSample,InputDataFormats="CB");

Denote $\mathbf{X}=(x,y)$ and specify the basis functions ${\mathbf{e}}_1 (x,y)=1,{\mathbf{e}}_2 (x,y)=x^2 ,{\mathbf{e}}_3 (x,y)=xy,{\mathbf{e}}_4 (x,y)=y^2$. Note that the linear functions $(x,y)\to x$ and $(x,y)\to y$ are not included as the linear terms in $\mathbf{f}$ are already known.

e1 = @(X) ones(1,size(X,2));
e2 = @(X) X(1,:).^2;
e3 = @(X) X(1,:).*X(2,:);
e4 = @(X) X(2,:).^2;
E = @(X) [e1(X); e2(X); e3(X); e4(X)];

Evaluate the basis functions at the sample points.

EEval = E(XSample);

Sequentially solve ${\mathbf{h}}_{\theta } =\mathbf{W}\mathbf{E}$ for 10 iterations, and set terms with absolute value less that $0.05$ to $0$.

iters = 10;
threshold = 0.1;
Ws = cell(iters,1);
W = hEval/EEval;
Ws{1} = W;
for iter = 2:iters
    belowThreshold = abs(W)<threshold;
    W(belowThreshold) = 0;
    for i = 1:size(W,1)
        aboveThreshold_i = ~belowThreshold(i,:);
        W(i,aboveThreshold_i) = hEval(i,:)/EEval(aboveThreshold_i,:);
    end
    Ws{iter} = W;
end

Display the identified equation.

Widentified = Ws{end};
fprintf(...
    "Identified dx/dt = %.2f + %.2f x^2 + %.2f xy + %.2f y^2 + %.2f x^3 + %.2f x^2 y + %.2f xy^2 + %.2f y^3 \n", ...
    Widentified(1,1), Widentified(1,2), Widentified(1,3), Widentified(1,4));

Identified dx/dt = 0.00 + 0.00 x^2 + -0.58 xy + 0.00 y^2 + 

fprintf(...
    "Identified dy/dt = %.2f + %.2f x^2 + %.2f xy + %.2f y^2 + %.2f x^3 + %.2f x^2 y + %.2f xy^2 + %.2f y^3 \n", ...
    Widentified(2,1), Widentified(2,2), Widentified(2,3), Widentified(2,4));

Identified dy/dt = 0.00 + 0.00 x^2 + 0.39 xy + 0.00 y^2 + 

Test the identified model

Now use $\hat{\mathbf{h}} (\mathbf{X})=\mathbf{W}\mathbf{E}(\mathbf{X})$ in place of ${\mathbf{h}}_{\theta }$ and solve the ODE.

function dXdt = identifiedModel(X,W,E)
x = X(1);
y = X(2);
% Known terms
dxdt = x;
dydt = -y;
% Identified terms
EEval = E(X);
WE = W*EEval;
dXdt = [dxdt; dydt];
dXdt = dXdt + WE;
end

Fidentified = ode(...
    ODEFcn = @(t,X) identifiedModel(X,W,E),...
    InitialValue = X0);
S = solve(Fidentified,ts);
scatter(ts,X(1,:),'b.');
hold on
scatter(ts,X(2,:),'r.');
plot(S.Time,S.Solution,'--')
title("Predicted v.s. True dynamics");
legend(["True x", "True y","Predicted x", "Predicted y"]);
hold off

Helper Functions

function x = softplusActivation(x)
x = max(x,0) + log(1 + exp(-abs(x)));
end

References

[1] Christopher Rackauckas, Yingbo Ma, Julius Martensen, Collin Warner, Kirill Zubov, Rohit Superkar, Dominic Skinner, Ali Ramadhan, and Alan Edelman. "Universal Differential Equations for Scientific Machine Learning". Preprint, submitted January 13, 2020. https://arxiv.org/abs/2001.04385

[2] Steven L. Brunton, Joshua L. Proctor, and J. Nathan Kutz. "Discovering governing equations from data by sparse identification of nonlinear dynamical systems". Proceedings of the National Academy of Sciences, 113 (15) 3932-3937, March 28, 2016.