Conjectural formulas for cluster variables of star graphs

Let SnS_n be the star graph on nn vertices, with central vertex labeled by 11, and let

C={{3},{4},,{n},{1,3,4,,n}}.\mathcal{C}=\{\{3\},\{4\},\dots,\{n\},\{1,3,4,\dots,n\}\}.

For a vertex subset SS, write YSY_S for the associated cluster-algebra element and [n]={1,,n}[n]=\{1,\dots,n\}. The cluster is C\mathcal{C}, and the formulas below conjecturally describe YSY_S for every vertex subset SS. Star-graph cluster formula conjecture. If 1S1\in S, then

YS={iS\{1,2}Yi2Y[n]\{1,2}(iSj1,2,iYj)+Y[n]i[n]\SYi+Y[n]Y[n]\{2}(i[n]\S1Yi),2S,Y[n]\{2}i[n\(S{2})]Yi+(iS\{1}Yi)(j[n]\(S{2})1Yi),2S.Y_S=\begin{cases} \displaystyle\frac{\prod\limits_{i\in S\backslash\{1,2\}}Y_i^2}{Y_{[n]\backslash\{1,2\}}}\left(\sum\limits_{i\notin S}\prod\limits_{j\ne1,2,i}Y_j\right)+\frac{Y_{[n]}}{\prod\limits_{i\in[n]\backslash S}Y_i}+\frac{Y_{[n]}}{Y_{[n]\backslash\{2\}}}\left(\sum\limits_{i\in[n]\backslash S}\frac{1}{Y_i}\right),&2\notin S,\\[2mm] \displaystyle\frac{Y_{[n]\backslash\{2\}}}{\prod\limits_{i\in[n\backslash(S\cup\{2\})]}Y_i}+\left(\prod\limits_{i\in S\backslash\{1\}}Y_i\right)\left(\sum\limits_{j\in[n]\backslash(S\cup\{2\})}\frac{1}{Y_i}\right),&2\in S. \end{cases}

If 1S1\notin S, then SS consists either of disconnected leaves or of a single leaf, and

YS={Yi,S={i},iSYi,otherwise.Y_S=\begin{cases}Y_i,&S=\{i\},\prod\limits_{i\in S}Y_i,&\text{otherwise.}\end{cases}

Here the source uses textrmif\operatorname{textrm}{if} and contains an apparent index mismatch in the second case, with YiY_i occurring inside a sum indexed by jj; these expressions are retained mathematically as stated. The conjecture proposes explicit formulas for cluster-algebra elements associated with subsets of a star graph and is presented as a future direction for extending rooted-cluster results to other clusters. No resolution is supplied in the source.

Sources & referencesView supporting material

Primary source

Esther Banaian, Sunita Chepuri, Elizabeth Kelley and Sylvester W. Zhang, “Rooted Clusters for Graph LP Algebras”, arXiv:2107.14785 (2022).

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