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In their article "On n-Gorenstein rings and Auslander rings of low injective dimension" Fuller and Iwanaga gave a homological characterisation of 2-Gorenstein Nakayama algebras with global dimension at most three, see theorem 3.16. there. Now Nakayama algebras (we always assume they are acyclic) are in natural bijection to Dyck paths. Call a Dyck path nice in case the corresponding Nakayama algebra is 2-Gorenstein with global dimension at most 3, see below for an elementary combinatoria description. I noticed with the computer that nice Dyck paths seem to be enumerated by $2^{n-2}$ (thats why I call them nice) and the subclass of nice Dyck paths with global dimension at most two by the Fibonacci numbers. This leads to the following question:

Question 1: Is there is a bijective proof mapping nice Dyck paths to some known/nice combinatorial objects?

Furthermore, to every nice Dyck path there is associated a canonical bijection and I wonder what this bijection is (there is a motivation to call this bijection homological rowmotion as it generalises the classical rowmotion from certain posets to more general combinatorial objects such as certain Dyck paths).

Question 2: What is the associated bijection to a nice Dyck path?

I currently have no elementary description so question 2, is more of a guess from the data what it might be.

An $n$-Kupisch series (which we can identify with a Dyck path via its area sequence) is a list of $n$ numbers $c:=[c_1,c_2,...,c_n]$ with $c_n=1$, $c_i \ge 2$ for $i \neq n$ and $c_i-1 \leq c_{i+1}$ for all $i=1,...,n-1$ and setting $c_0:=c_n$. The number of such $n$-Kupisch series is equal to $C_{n-1}$ (Catalan numbers).

Here are some examples of the nice Dyck paths for small $n$ together with the bijection on $\{1,..,n\}$.

$n=2$:

   [ [ 2, 1 ], [ [ 1, 2 ], [ 2, 1 ] ] ] 

$n=3$:

  [ [ 2, 2, 1 ], [ [ 1, 3 ], [ 2, 1 ], [ 3, 2 ] ] ], 

  [ [ 3, 2, 1 ], [ [ 1, 2 ], [ 2, 3 ], [ 3, 1 ] ] ] 

n=4:

   [ [ 2, 2, 2, 1 ], [ [ 1, 4 ], [ 2, 1 ], [ 3, 2 ], [ 4, 3 ] ] ], 

   [ [ 3, 2, 2, 1 ], [ [ 1, 2 ], [ 2, 4 ], [ 3, 1 ], [ 4, 3 ] ] ], 

   [ [ 2, 3, 2, 1 ], [ [ 1, 3 ], [ 2, 1 ], [ 3, 4 ], [ 4, 2 ] ] ], 

   [ [ 4, 3, 2, 1 ], [ [ 1, 2 ], [ 2, 3 ], [ 3, 4 ], [ 4, 1 ] ] 

n=5:

   [ [ [ 3, 2, 2, 2, 1 ], [ [ 1, 2 ], [ 2, 5 ], [ 3, 1 ], [ 4, 3 ], [ 5, 4 ] ] ],

   [ [ 2, 3, 2, 2, 1 ], [ [ 1, 3 ], [ 2, 1 ], [ 3, 5 ], [ 4, 2 ], [ 5, 4 ] ] ],

   [ [ 4, 3, 2, 2, 1 ], [ [ 1, 2 ], [ 2, 3 ], [ 3, 5 ], [ 4, 1 ], [ 5, 4 ] ] ],

   [ [ 2, 2, 3, 2, 1 ], [ [ 1, 4 ], [ 2, 1 ], [ 3, 2 ], [ 4, 5 ], [ 5, 3 ] ] ],

   [ [ 3, 2, 3, 2, 1 ], [ [ 1, 2 ], [ 2, 4 ], [ 3, 1 ], [ 4, 5 ], [ 5, 3 ] ] ],

   [ [ 3, 3, 3, 2, 1 ], [ [ 1, 5 ], [ 2, 4 ], [ 3, 1 ], [ 4, 2 ], [ 5, 3 ] ] ],

   [ [ 2, 4, 3, 2, 1 ], [ [ 1, 3 ], [ 2, 1 ], [ 3, 4 ], [ 4, 5 ], [ 5, 2 ] ] ],

   [ [ 5, 4, 3, 2, 1 ], [ [ 1, 2 ], [ 2, 3 ], [ 3, 4 ], [ 4, 5 ], [ 5, 1 ] ] ] 

n=6:

   [ [ 2, 3, 2, 2, 2, 1 ], [ [ 1, 3 ], [ 2, 1 ], [ 3, 6 ], [ 4, 2 ], [ 5, 4 ], [ 6, 5 ] ] ], 

   [ [ 4, 3, 2, 2, 2, 1 ], [ [ 1, 2 ], [ 2, 3 ], [ 3, 6 ], [ 4, 1 ], [ 5, 4 ], [ 6, 5 ] ] ],

   [ [ 2, 2, 3, 2, 2, 1 ], [ [ 1, 4 ], [ 2, 1 ], [ 3, 2 ], [ 4, 6 ], [ 5, 3 ], [ 6, 5 ] ] ], 

   [ [ 3, 2, 3, 2, 2, 1 ], [ [ 1, 2 ], [ 2, 4 ], [ 3, 1 ], [ 4, 6 ], [ 5, 3 ], [ 6, 5 ] ] ],

   [ [ 2, 4, 3, 2, 2, 1 ], [ [ 1, 3 ], [ 2, 1 ], [ 3, 4 ], [ 4, 6 ], [ 5, 2 ], [ 6, 5 ] ] ],

   [ [ 5, 4, 3, 2, 2, 1 ], [ [ 1, 2 ], [ 2, 3 ], [ 3, 4 ], [ 4, 6 ], [ 5, 1 ], [ 6, 5 ] ] ], 

   [ [ 3, 2, 2, 3, 2, 1 ], [ [ 1, 2 ], [ 2, 5 ], [ 3, 1 ], [ 4, 3 ], [ 5, 6 ], [ 6, 4 ] ] ],

   [ [ 2, 3, 2, 3, 2, 1 ], [ [ 1, 3 ], [ 2, 1 ], [ 3, 5 ], [ 4, 2 ], [ 5, 6 ], [ 6, 4 ] ] ],

   [ [ 4, 3, 2, 3, 2, 1 ], [ [ 1, 2 ], [ 2, 3 ], [ 3, 5 ], [ 4, 1 ], [ 5, 6 ], [ 6, 4 ] ] ], 

   [ [ 3, 3, 3, 3, 2, 1 ], [ [ 1, 5 ], [ 2, 6 ], [ 3, 1 ], [ 4, 2 ], [ 5, 3 ], [ 6, 4 ] ] ],

   [ [ 4, 3, 3, 3, 2, 1 ], [ [ 1, 2 ], [ 2, 6 ], [ 3, 5 ], [ 4, 1 ], [ 5, 3 ], [ 6, 4 ] ] ],

   [ [ 2, 2, 4, 3, 2, 1 ], [ [ 1, 4 ], [ 2, 1 ], [ 3, 2 ], [ 4, 5 ], [ 5, 6 ], [ 6, 3 ] ] ],

   [ [ 3, 2, 4, 3, 2, 1 ], [ [ 1, 2 ], [ 2, 4 ], [ 3, 1 ], [ 4, 5 ], [ 5, 6 ], [ 6, 3 ] ] ],

   [ [ 3, 3, 4, 3, 2, 1 ], [ [ 1, 5 ], [ 2, 4 ], [ 3, 1 ], [ 4, 2 ], [ 5, 6 ], [ 6, 3 ] ] ],

   [ [ 2, 5, 4, 3, 2, 1 ], [ [ 1, 3 ], [ 2, 1 ], [ 3, 4 ], [ 4, 5 ], [ 5, 6 ], [ 6, 2 ] ] ],

   [ [ 6, 5, 4, 3, 2, 1 ], [ [ 1, 2 ], [ 2, 3 ], [ 3, 4 ], [ 4, 5 ], [ 5, 6 ], [ 6, 1 ] ] ] ]

In the following I give the elemenatary combinatorial description of nice Dyck paths. Sadly, it is quite complicated at the moment despite the possibly very nice enumeration.

I found the following combinatorial characterisation of those Dyck paths (compare with Combinatorics problem related to Motzkin numbers with prize money I ):

The CoKupisch series $d$ of $c$ is defined as $d=[d_1,d_2,...,d_n]$ with $d_i:= \min \{k | k \geq c_{i-k} \} $ and $d_1=1$. One can show that the $d_i$ are a permutation of the $c_i$. A number $a \in \{1,...,n \}$ is a descent if $a=1$ or $c_a >c_{a-1}$. Define a corresponding set, indexed by descents: $X_1 := \{1,2,...,c_1-1 \}$, and $X_a := \{ c_{a-1}, c_{a-1}+1 ,..., c_a -1 \}$ for descents $a > 1$.

A $n$-Kupisch series is called $2-$Gorenstein if it satisfies the following condition:

  1. condition: for each descent $a$, and each $b \in X_a$: either $c_{a+b} \geq c_{a+b-1}$ or $d_{a+b-1} = d_{a+b + c_{a+b}-1} - c_{a+b}$ is satisfied.

Now an $n$-Kupisch path is nice if and only if it is 2-Gorenstein and it has global dimension at most 3. Sadly there is no nice formal description of global dimension at most 3 but it can be pictured in a nice way in a Dyck path.

Call an $i$ with $1 \leq i \leq n-1$ good in case one of the following three conditions hold:

  • $c_{i+1}=c_i -1$ (equivalent to the simple module $S_i$ having projective dimension one)

  • ($c_{i+1}>c_i-1 $ and $c_{i+c_i}=c_{i+1}-c_i+1$) (equivalent to $S_i$ having projective dimension two)

  • ($c_{i+1}>c_i-1 $ and $c_{i+c_i}>c_{i+1}-c_i+1$ and $c_{i+c_{i+1}+1}=c_{i+c_i}-c_{i+1}+c_i-1$) (equivalent to $S_i$ having projective dimension three)

Now the 2. condition is:

  1. condition: Every $i$ with $1 \leq i \leq n-1$ is good.

So an n-Kupisch series (=Dyck path) is nice if and only if it satisfies condition 1. and 2.

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This is a conjectural answer.

Let $w = 0\dots01$ be a binary word of length $n$. Then $\phi(w)$ is the Dyck path $U^{(n+1)/2} (UD)^{(n-1)/2} D^{(n+1)/2}$ if $n$ is odd and $U^{n/2} (UD)^{n/2} D^{n/2}$ if $n$ is even.

Let $w = 0^{n_1} 1 0^{n_2} 1 \dots 0^{n_k} 1$ be any binary word ending with a $1$. Then $\phi(w) = \phi(0^{n_1} 1) \phi(0^{n_2} 1)\dots \phi(0^{n_k} 1)$.

Finally, to obtain the nice Dyck path, apply the Lalanne-Kreweras involution https://www.findstat.org//Mp00120.

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    $\begingroup$ I don't know what to make of all these answers by FindStat, but I think the correct link for this answer is findstat.org/MapsDatabase/Mp00120 $\endgroup$ – David White Jul 19 '20 at 18:16
  • $\begingroup$ @DavidWhite Thank you for noticing it! The initial link did not work, I changed it and checked, but now my version does not work either, don't know why. $\endgroup$ – მამუკა ჯიბლაძე Jul 19 '20 at 19:28
  • $\begingroup$ Seems to work now $\endgroup$ – მამუკა ჯიბლაძე Jul 19 '20 at 19:36
  • $\begingroup$ @DavidWhite there should be a redirect, for me both versions work. I use the shorter version, because it is shorter. Concerning "FindStat answers": I like to post answers under this name if my scripts find the answer essentially automatically. $\endgroup$ – Martin Rubey Jul 19 '20 at 19:58
  • $\begingroup$ @MartinRubey, I think the issue is that the other user edited to change your // to / in the URL. Also, I didn't mean to be accusatory about the FindStat user, but at first glance I thought it odd to see so many links to the same domain. Instead of flagging as spam, I followed one and learned about your project, which seemed legit. I leave it to the experts to decide if it answers the present question. $\endgroup$ – David White Jul 19 '20 at 20:06

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