Generalized Pfaffian product formulas for Motzkin triangle entries

From papers

Let nn and kk be positive integers. Define the Motzkin-triangle entries by

Mi(k)=( ⁣i1k1 ⁣)2F1 ⁣(ki2,ki+12k+1;4).\mathcal{M}^{(k)}_{i}=\left(\!\genfrac{}{}{0pt}{}{i-1}{k-1}\!\right){}_2F_1\!\left(\genfrac{}{}{0pt}{}{\frac{k-i}{2},\frac{k-i+1}{2}}{k+1};4\right).

For a 2n×2n2n\times 2n skew-symmetric matrix, let Pf\operatorname{Pf} denote its Pfaffian. Generalized Motzkin Pfaffian conjecture. The Pfaffian

Pf ⁣((ji)Mi+j2(k))1i,j2n\operatorname{Pf}\!\left((j-i)\mathcal{M}^{(k)}_{i+j-2}\right)_{1\leqslant i,j\leqslant 2n}

equals

i=0m1j=0k1(4ki+2j+k)\prod_{i=0}^{m-1}\prod_{j=0}^{k-1}(4ki+2j+k)

if m=n/km=n/k is an integer, and equals

(j=1k/212jk)(i=0m1j=1k(4ki+2jk))\left(\prod_{j=1}^{\lfloor k/2\rfloor}\frac{1}{2j-k}\right)\left(\prod_{i=0}^{m-1}\prod_{j=1}^{k}(4ki+2j-k)\right)

if kk is odd and m=(n+k/2)/km=(n+\lfloor k/2\rfloor)/k is an integer; it is zero in all other cases. Meanwhile,

Pf ⁣((ji)(Mi+j2(k)+Mi+j1(k)))1i,j2n\operatorname{Pf}\!\left((j-i)\left(\mathcal{M}^{(k)}_{i+j-2}+\mathcal{M}^{(k)}_{i+j-1}\right)\right)_{1\leqslant i,j\leqslant 2n}

equals

i=0m1j=0k1(4ki+2j+k+1)\prod_{i=0}^{m-1}\prod_{j=0}^{k-1}(4ki+2j+k+1)

if m=n/km=n/k is an integer, and equals

(j=1k/212jk1)(i=0m1j=1k(4ki+2jk1))\left(\prod_{j=1}^{k/2}\frac{1}{2j-k-1}\right)\left(\prod_{i=0}^{m-1}\prod_{j=1}^{k}(4ki+2j-k-1)\right)

if kk is even and m=(n+k/2)/km=(n+k/2)/k is an integer; it is zero in all other cases. The formulas generalize the preceding Pfaffian evaluation for the Motzkin numbers and would provide product evaluations for broad families of Pfaffians built from Motzkin-triangle entries. The source does not provide evidence resolving this proposed generalization.

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Sources & referencesView supporting material

Primary source

Masao Ishikawa and Christoph Koutschan, “Zeilberger's Holonomic Ansatz for Pfaffians”, arXiv:1201.5253 (2012).

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