This question was tagged : Numbers on the blackboard: From 2-2015 to 1-2014

Currently, that is the only question under this tag, but these questions would also fit under what I think the intention of the tag is:

This type of problem is characterized by a set of numbers being written on a blackboard, and operations being used to change the numbers. The solution usually involves an invariant or monovariant of the system. As xnor commented:

I want to point out the general idea here that also solves this problem: apply a transformation to the numbers so that the combining operation on the transformed numbers is commutative and associative.

Two of the questions are also tagged : "A puzzle on a discrete system where one has to decide whether a certain system state can be reached through a finite number of steps." This seems like the closest tag we have currently, but it is broader than the category of "blackboard questions".

Should be a tag of its own, a synonym of , or neither?

  • 2
    $\begingroup$ Blackboard should not a synonym of reachability, since the two categories can't be used interchangeably. $\endgroup$ Oct 21, 2015 at 3:29
  • $\begingroup$ What is a blackboard puzzle? $\endgroup$
    – user20
    Oct 21, 2015 at 15:28
  • $\begingroup$ @Emrakil: A puzzle in which various numbers are written on a board, and they can be erase and replaced with other numbers based on various rules; the goal is to reach a certain point or prove that it's impossible. $\endgroup$
    – Deusovi Mod
    Oct 21, 2015 at 15:38
  • $\begingroup$ @MikeEarnest I was under the impression that a non-notable subset of a category of puzzles can be made a synonym of the larger category. For example, party-security is a synonym of password, and minimum-solve is a synonym of optimization. In both of these cases, the synonym is a subset of the main tag, but they aren't interchangeable. $\endgroup$
    – f''
    Oct 21, 2015 at 17:52

1 Answer 1


First of all, I don't think they should be synonyms. We can leave the tag as it is, because there are a few such questions and more might turn up in the future.

Yes, it is a subset of but how is that a problem? There are reachability questions that do not involve a blackboard also. And is a subset of and both are very popular.


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