Fibonacci recurrence-coefficient conjecture for 2-distant and 3-distant noncrossing partitions

Let NCPk(n)NCP_k(n) denote the number of kk-distant noncrossing partitions of [n][n]. For a moment sequence (μn)n0(\mu_n)_{n\geq 0}, write its associated monic orthogonal polynomials in the form

Pn+1(x)=(xbn)Pn(x)λnPn1(x).P_{n+1}(x)=(x-b_n)P_n(x)-\lambda_nP_{n-1}(x).

Here FnF_n denotes the nn-th Fibonacci number, defined by Fn+1=Fn+Fn1F_{n+1}=F_n+F_{n-1} and F1=F2=1F_1=F_2=1.

Fibonacci recurrence-coefficient conjecture. If μn=NCP2(n)\mu_n=NCP_2(n), then b0=b1=λ1=1b_0=b_1=\lambda_1=1, and for n2n\geq2,

bn=31F2n1F2n3,λn=1+1(F2n3)2.b_n=3-\frac{1}{F_{2n-1}F_{2n-3}},\qquad \lambda_n=1+\frac{1}{(F_{2n-3})^2}.

If μn=NCP3(n)\mu_n=NCP_3(n), then b0=b1=λ1=1b_0=b_1=\lambda_1=1, λ2=2\lambda_2=2, and, for n3n\geq3, bn=3b_n=3 and λn=2\lambda_n=2.

The conjecture gives explicit recurrence coefficients for the orthogonal polynomials associated with the enumerative sequences of 2-distant and 3-distant noncrossing partitions. The preceding computations provide evidence for these formulas, but the source does not state a resolution, so the conjecture remains open.

Sources & referencesView supporting material

Primary source

Dan Drake and Jang Soo Kim, “k-distant crossings and nestings of matchings and partitions”, arXiv:0812.2725 (2009).

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