v0.1:

  * Added: Theorem on positivity of solutions.

  * Added: Reference to [CJLL20]

  * Added: Theorem on decay rate is now more general.

  * Fixed: Clarified the discussion in point 2-2 of the proof of the theorem
           on decay.

  * Removed: open problem about positivity of solutions.

  * Fixed: Format of named theorems.

  * Fixed: Minor formatting fixes.

  * Fixed: In proof of decay: indenting error.