# “Phase change” of a purely mathematical system

Every so often I hear people talking about "phase transitions" in purely mathematical or computer-science contexts, where there is no physics in sight. Today, for example, I heard some people talking about "phase transitions" when coloring edges in random graphs.

Is this just a simple reuse of the physics term, or is there a more general mathematical concept of a "phase change", of which physical phase changes are one instance? If so, what are some illustrative purely mathematical/nonphysical examples?

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I think there is a connection via statistical mechanics, Ising models, percolation theory, etc. My naive impression is that there is a mathematical concept of phase transition (roughly speaking when the behavior of a system depends discontinuously on some continuous parameter) of which physical phase transitions are an instance. – Qiaochu Yuan Apr 24 '12 at 22:27
@QiaochuYuan Yeah, I think so too. I'm hoping someone knowledgeable about those fields could elaborate or give references on how to make the connection precise. – Nick Alger Apr 24 '12 at 23:40

In Where the Really Hard Problems Are the concept of phase change in NP complete problems is discussed. It has the sense of a narrow boundary region from physics, but not entirely the sense of fixing things (like the temperature of a melting solution).

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Ahh, that seems to contain a discussion of two of the examples that I've run across. It seems like more of an analogy than a direct correspondence though. There's no analogs of "entropy", "temperature", "free energy", or other quantities you would expect in thermodynamic systems. :( – Nick Alger Apr 23 '12 at 22:26

The most common example I can think of is in random graph theory, which serves as the mathematical model of all sorts of networks, where an abrupt transition occurs between the disconnected regime and the connected regime when the probability of the existence of an edge between two nodes exceeds $\log n/n$, where $n$ is the number of nodes (vertices). If calling that a "phase change" is an abuse, then so be it.