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Nov 26, 2023 at 14:24 comment added Kai oeis.org/A000975
Nov 26, 2023 at 12:08 comment added Kai The wording is fine, my thinking wasn't. I interpret your signatures roughly as "what's on the stack of a PDA for Dyck$(2)$". We ought to exclude those signatures $x$ for which there's no matching $y\in A^n$. If I'm not mistaken, they are the $x$ with an odd $n - |x|$. Sadly, I'm not aware of a closed form for $\sum_{i+j\le n\\ n-i-j\bmod 2 = 0}{i+j\choose i}$.
Nov 24, 2023 at 22:08 comment added D.W. @Kai, sorry for the imprecise wording. That's not a lower bound. There are strings in $\{[,(\}^{\le n}$ that are not a possible signature. e.g., for $n=3$, $[($ is not a possible signature, i.e., there is no string $x$ of length 3 whose signature is $[($. Consequently, $2^{n+1}-1$ is an upper bound on the number of different signatures but not a lower bound.
Nov 24, 2023 at 22:07 history edited D.W. CC BY-SA 4.0
Clarify definition of N
Nov 23, 2023 at 11:30 comment added Kai Looks like you've already done all the work for the lower bound being $\lg (2^{n+1}-1)$, too.
Nov 23, 2023 at 8:41 history answered D.W. CC BY-SA 4.0