# Tensors, Tensors, Tensors

This post is going to take a step back from some of the machine learning topics that I've been writing about recently and go back to some basics: math! In particular, tensors. This is a topic that is casually mentioned in machine learning papers but for those of us who weren't physics or math majors (*cough* computer engineers), it's a bit murky trying to understand what's going on. So on my most recent vacation, I started reading a variety of sources on the interweb trying to piece together a picture of what tensors were all about. As usual, I'll skip the heavy formalities (partly because I probably couldn't do them justice) and instead try to explain the intuition using my usual approach of examples and more basic maths. I'll sprinkle in a bunch of examples and also try to relate it back to ML where possible. Hope you like it!

# Residual Networks

Taking a small break from some of the heavier math, I thought I'd write a post (aka learn more about) a very popular neural network architecture called Residual Networks aka ResNet. This architecture is being very widely used because it's so simple yet so powerful at the same time. The architecture's performance is due its ability to add hundreds of layers (talk about deep learning!) without degrading performance or adding difficulty to training. I really like these types of robust advances where it doesn't require fiddling with all sorts of hyper-parameters to make it work. Anyways, I'll introduce the idea and show an implementation of ResNet on a few runs of a variational autoencoder that I put together on the CIFAR10 dataset.

# Variational Autoencoders with Inverse Autoregressive Flows

In this post, I'm going to be describing a really cool idea about how to improve variational autoencoders using inverse autoregressive flows. The main idea is that we can generate more powerful posterior distributions compared to a more basic isotropic Gaussian by applying a series of invertible transformations. This, in theory, will allow your variational autoencoder to fit better by concentrating the stochastic samples around a closer approximation to the true posterior. The math works out so nicely while the results are kind of marginal [1]. As usual, I'll go through some intuition, some math, and have an implementation with few experiments I ran. Enjoy!

# Autoregressive Autoencoders

You might think that I'd be bored with autoencoders by now but I still find them extremely interesting! In this post, I'm going to be explaining a cute little idea that I came across in the paper MADE: Masked Autoencoder for Distribution Estimation. Traditional autoencoders are great because they can perform unsupervised learning by mapping an input to a latent representation. However, one drawback is that they don't have a solid probabilistic basis (of course there are other variants of autoencoders that do, see previous posts here, here, and here). By using what the authors define as the autoregressive property, we can transform the traditional autoencoder approach into a fully probabilistic model with very little modification! As usual, I'll provide some intuition, math and an implementation.

# Semi-supervised Learning with Variational Autoencoders

In this post, I'll be continuing on this variational autoencoder (VAE) line of exploration (previous posts: here and here) by writing about how to use variational autoencoders to do semi-supervised learning. In particular, I'll be explaining the technique used in "Semi-supervised Learning with Deep Generative Models" by Kingma et al. I'll be digging into the math (hopefully being more explicit than the paper), giving a bit more background on the variational lower bound, as well as my usual attempt at giving some more intuition. I've also put some notebooks on Github that compare the VAE methods with others such as PCA, CNNs, and pre-trained models. Enjoy!