TODO what it means to solve an Ising model in general?

stanford.edu/~jeffjar/statmech/lec4.html gives some good notions:

- $<σ_{i}>$ is the expectation value of the value. It is therefore a number between -1.0 an and 1.0, -1.0 means everything is always down, 0.0 means half up half down, and 1.0 means all up
- $<σ_{i}σ_{j}>$: correlation between neighboring states. TODO.