We study proof techniques for bisimilarity based on unique solution of equations. We draw inspiration from a result by Roscoe in the denotational setting of CSP and for failure semantics, essentially stating that an equation (or a system of equations) whose infinite unfolding never produces a divergence has the unique-solution property. We transport this result onto the operational setting of CCS and for bisimilarity. We then exploit the operational approach to: refine the theorem, distinguishing between different forms of divergence; derive an abstract formulation of the theorems, on generic LTSs; adapt the theorems to other equivalences such as trace equivalence, and to preorders such as trace inclusion. We compare the resulting techniques to enhancements of the bisimulation proof method (the `up-to techniques'). Finally, we study the theorems in name-passing calculi such as the asynchronous $\pi$-calculus, and use them to revisit the completeness part of the proof of full abstraction of Milner's encoding of the $\lambda$-calculus into the $\pi$-calculus for L\'evy-Longo Trees. Comment: This is an extended version of the paper with the same title published in the proceedings of CONCUR'17

@article{paperbot773,
  title = {Divergence and unique solution of equations},
  author = {Adrien Durier and Daniel Hirschkoff and Davide Sangiorgi},
  journal = {Logical Methods in Computer Science},
  volume = {Volume 15, Issue 3},
  year = {2019},
  doi = {10.23638/lmcs-15(3:12)2019}
}