We show how the success of deep learning could depend not only on mathematics but also on physics: although well-known mathematical theorems guarantee that neural networks can approximate arbitrary functions well, the class of functions of practical interest can frequently be approximated through {\textquotedblleft}cheap learning{\textquotedblright} with exponentially fewer parameters than generic ones. We explore how properties frequently encountered in physics such as symmetry, locality, compositionality, and polynomial log-probability translate into exceptionally simple neural networks. We further argue that when the statistical process generating the data is of a certain hierarchical form prevalent in physics and machine learning, a deep neural network can be more efficient than a shallow one. We formalize these claims using information theory and discuss the relation to the renormalization group. We prove various {\textquotedblleft}no-flattening theorems{\textquotedblright} showing when efficient linear deep networks cannot be accurately approximated by shallow ones without efficiency loss; for example, we show that *n* variables cannot be multiplied using fewer than 2*n* neurons in a single hidden layer.

Although\ there\ is\ growing\ interest\ in\ measuring\ integrated\ information\ in\ computational\ and cognitive systems, current methods for doing so in practice are computationally unfeasible.\ Existing and\ novel\ integration\ measures\ are\ investigated\ and\ classified\ by\ various\ desirable\ properties.\ \ A simple taxonomy of Φ-measures is presented where they are each characterized by their choice of factorization\ method\ (5\ options),\ choice\ of\ probability\ distributions\ to\ compare\ (3 {\texttimes} 4\ options) and choice of measure for comparing probability distributions (7 options).\ When requiring the Φ- measures to satisfy a minimum of attractive properties, these hundreds of options reduce to a mere handful, some of which turn out to be identical.\ Useful exact and approximate formulas are derived that can be applied to real-world data from laboratory experiments without posing unreasonable computational demands.

}, doi = {10.1371/journal.pcbi.100512310.1371}, url = {http://dx.plos.org/10.1371/journal.pcbi.1005123}, author = {Tegmark, Max}, editor = {Seth, Anil} }