Abstract
When proving theorems, we have a luxury that is so fundamental, we often take it for granted: If we need to work with an object, we say “let x be \(\langle\) the object in question\(\rangle\)” and we move on with the proof. Especially in the finite setting it is obvious that, given enough patience, we should be able to find the object: Simply try out all possibilities and, if there is an object as desired, at least one of them will work. As long as we are not interested in the object itself, this approach is very efficient for developing a theory.
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- 1.
This is a possible trap when implementing algorithms or analyzing their complexity. Simple-looking steps can become quite complex and orders of magnitude of the actual complexity can be overlooked because they are hidden in sub-steps whose length does depend on the input size.
- 2.
Generating every possible order-preserving self-map and checking it for a fixed point would be an exceedingly naive idea.
- 3.
Again, there will be no confusion with the use of the letter \(\mathcal{C}\) here.
- 4.
The ambiguity resulting from specification of constraints for sets {x i ,x j } of variables (the order of the variables matters in the specification of C ij ) is removed by demanding i < j. The alternative, which makes things unnecessarily technical, would be to specify constraints for ordered pairs of variables and demand the appropriate symmetry for the constraints C ij and C ji.
- 5.
Careful with notation here. In some works, an instantiation is called consistent if (ai ,a j )∉C ij, which is consistent with a constraint being something that forbids configurations.
- 6.
For our purposes we can always assume that the order of the variables x 1, …, x n is the order of the indices. We will not explicitly specify the order of the value domains.
- 7.
I have seen a colleague, who was not a mathematician, use a similarly inefficient approach in his research. This is another instance that shows how mathematicians can help in collaborations.
- 8.
Personally, I prefer forward checking over backtracking and I don’t think I’m the only one.
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Schröder, B. (2016). Constraint Satisfaction Problems. In: Ordered Sets. Birkhäuser, Cham. https://doi.org/10.1007/978-3-319-29788-0_5
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