When studying representation theory, special functions or various other topics one is very likely to encounter the following identity at some point:
$$det left(frac{1}{x _i+y _j}right) _{1le i,j le n}=frac{prod _{1le i < jle n} (x _j-x _i)(y _j-y _i)}{prod _{i,j=1}^n (x _i+y _j)}$$
This goes under the name of Cauchy's determinant identity and has various generalizations and analogous statements. There is also a lot of different proofs using either analysis or algebra. In my case I have always seen it introduced (or motivated) as an identity that plays an important role in combinatorics, but I realized that I haven't really seen this identity in a combinatorial context before. In this question I'm asking for a combinatorial interpretation of the above identity. A bonus to someone who can give such an interpretation to Borchardt's variation:
$$det left(frac{1}{(x _i+y _j)^2}right) _{1le i,j le n}=frac{prod _{1le i < jle n} (x _j-x _i)(y _j-y _i)}{prod _{i,j=1}^n (x _i+y _j)} cdot text{per}left(frac{1}{x _i +y _j}right) _{1 le i,j le n}$$
(This seems a little too ambitious though, and I would be happy to accept an answer of just the first question)
Saturday, 5 March 2016
co.combinatorics - What role does Cauchy's determinant identity play in combinatorics?
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