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Appearance of branched motifs in the spectra of $BC_N$ type Polychronakos spin chains

As is well known, energy levels appearing in the highly degenerate spectra of the $A_{N-1}$ type of Haldane-Shastry and Polychronakos spin chains can be classified through the motifs, which are characterized by some sequences of the binary digits like `0' and `1'. In a similar way, at present we classify all energy levels appearing in the spectra of the $BC_N$ type of Polychronakos spin chains with Hamiltonians containing supersymmetric analogue of polarized spin reversal operators. To this end, we show that the $BC_N$ type of multivariate super Rogers-Szegö (SRS) polynomials, which at a certain limit reduce to the partition functions of the later type of Polychronakos spin chains, satisfy some recursion relation involving a $q$-deformation of the elementary supersymmetric polynomials. Subsequently, we use a Jacobi-Trudi like formula to define the corresponding $q$-deformed super Schur polynomials and derive a novel expression for the $BC_N$ type of multivariate SRS polynomials as suitable linear combinations of the $q$-deformed super Schur polynomials. Such an expression for SRS polynomials leads to a complete classification of all energy levels appearing in the spectra of the $BC_N$ type of Polychronakos spin chains through the `branched' motifs, which are characterized by some sequences of integers of the form $(δ_1, δ_2,..., δ_{N-1}|l)$, where $δ_i \in \{ 0,1 \}$ and $ l \in \{ 0,1,...,N \}$. Finally, we derive an extended boson-fermion duality relation among the restricted super Schur polynomials and show that the partition functions of the $BC_N$ type of Polychronakos spin chains also exhibit similar type of duality relation.