Weakly maximal decidable structures
We prove that there exists a structure whose monadic second order theory is decidable, and such that the first-order theory of every expansion of by a constant is undecidable.
Tree inclusion problems
Given two trees (a target and a pattern ) and a natural number , consist in deciding whether occurs as an embedded subtree of and/or finding the number of size (at most) windows of which contain pattern as an embedded subtree. is an embedded subtree of if can be obtained by deleting some nodes from (if a node is deleted, all edges adjacent to are also deleted, and outgoing edges are replaced by edges going from the parent of (if it exists) to the children of ). Deciding whether ...
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