Adding supporting code for blog post

[mmarkows17]

Supporting code for the blog post Physics-Informed Machine Learning: Methods and Implementation

[bwdGitHub]

I would prefer the arguments here to be the other way around, fourierLayer(numModes, tWidth) and spectralConvolution1dLayer(numModes, tWidth). I think that has a better analogy to convolution1dLayer(filterSize, numFilters).

The numFilters and tWidth are analogous in that they control some hidden size of some linear operation with the weights. The numModes and filterSize are analogous in that they control the size/resolution of the convolution filter in some sense, and both can have N entries for an N-d conv or spectral conv.

[conordaly0]

+1 to Ben's comment about the ordering of the inputs!

[mmarkows17]

Changed it to (numModes, tWidth) in the most recent commit. Btw I got this from the doc example: https://www.mathworks.com/help/deeplearning/ug/solve-pde-using-fourier-neural-operator.html, should it be changed there too?

[bwdGitHub]

I expect we could use the same version here and in the 1d FNO example - maybe we should consider a directory of shared tools for the examples.

That's something we'll have to look into in future I think, not something to try deal with in this PR.

[conordaly0]

This is awesome Mae!

I still need to review the other MLX files, but I wanted to post my initial comments first.

[conordaly0]

I find it frustrating working with MLX files on GitHub because no-one can see the code. This also makes it hard to review.

My personal preference is to work locally in the MLX, then generate a markdown version which is added as a README, a plain M-code version so that the raw code is readable, and (optionally) the MLX file itself.

[conordaly0]

• $\omega_0$ is undefined in the intro.
• There's something wrong with this sentence "Using automatic differentiation, the enforces uses a custom loss function"
• There is a comment that once the network is trained predictions are made by numerically approximating Hamilton's equations. Is that true? The AD derivatives are exact. The value of the derivative is a numerical approximation, in the same way that computing, say, sin(2) in MATLAB is a numerical approximation <-- but we don't often refer to the latter as an approximation.
• Instead of transposing the inputs, just flip the labels. E.g. instead of inputs = dlarray([q'; p'],"BC"); we can write inputs = dlarray([q p],"CB");.

• • In 25a, and possibly earlier releases, it should be for the network to be unformatted -- i.e. to not have any labels on the data at all. I propose we do this. Let me know if I can help.

• Use the name modelLoss over modelGradients -- since the primary output of the function is the loss.
• "The original ode45 can't use dlarray..." it can! What's not supported are formatted arrays -- all the more reason to use an unformatted network.
• Prefer camel case over underscores, e.g. dqLoss or lossDq instead of loss_dq.
• I don't like the function name dlderivative because it's not functioning as a deep learning specific variant of a derivative function, in the same way that dlconv is a DL variant of conv. Also the function isn't really computing a derivative, it's taking a Hamiltonian $H$ and a set of $q-p$ inputs and returning $H_p$ and $H_q$. Looking at the code, it's almost always that $H_q$ isn't really what's needed -- instead it's $\dot{p}=-H_q$ So I think we should call the function something like computeDerivativesFromHamiltonian and make the function return $\dot{p}$ and $\dot{q}$. Since "compute" is kind of a redundant work we could even go with hamiltonianDerivatives or stateDerivatives?
• Instead of predmodel how about plain old model?

[mmarkows17]

Hi Conor, thank you so much, I made a bunch of these changes.

Regarding the formatting: I removed the formatting (I pushed these changes hopefully you can see them?) but now I have a bunch of transposes all over the place so not sure if I did this correctly. For example, in the function hamiltonianDerivatives (was dlderivative), I used to have
dH = dljacobian(H,qp,1). Now I have
dH = dljacobian(H,qp,2),
and I need to do
dHdq = dH(:,:,1)';
dHdp = dH(:,:,2)';
to compensate. Wasn't expecting them to become tensors? Maybe I should flatten them or something?

Further, when I go to make predictions by solving Hamilton's equations, I now need to transpose the x or else I get "was expecting input of size 2" error:
f = @(t,x) accOde(dlarray(t),dlarray(x'),net);

[mmarkows17]

Regarding "The original ode45 can't use dlarray..." -- I copied and pasted that from our HNN example on our Github: https://github.com/matlab-deep-learning/SciML-and-Physics-Informed-Machine-Learning-Examples/tree/main/hamiltonian-neural-network, which was created with 22b so it's good you are helping me update the code for new releases!

Do I still need to have the lines
accOde = dlaccelerate(@model); and
'f = @(t,x) accOde(dlarray(t),dlarray(x'),net);'
before calling ode45? I tried 'f = @(t,x) accOdet,x',net);' and while this worked, it was much slower than converting them to dlarrays first in the function handle.

[conordaly0]

Is it absolutely necessary to include this MAT-file (and the others) in the repo? Can the data be created on-the-fly?

We should avoid including data with the repo -- unless it is absolutely necessary for some reason. The last time I looked into this, every time the repo is versioned git records a copy of binary files. This causes the repo to become exorbitantly large and leads to a painful clone experience for users. I think this is particularly important for the SciML repo which houses lots of examples. If every example stored data or networks, the repo would quickly balloon in size and become unusable.

The best approach is for binaries -- whether it is data or saved networks -- to be stored on supportfiles. You can see how we've done that in other repos for with large binaries: https://github.com/matlab-deep-learning/resnet-50/blob/317ea186da1cb3d429d53559fed7386548ff8e47/assembleResNet50.m#L18 .

[conordaly0]

Is it necessary to work in a project? I see that it adds a load of xml files ot the repo.