Gauss polynomials and the rotation algebra. (English) Zbl 0665.46051

Choi, Man-Duen; Elliott, George A.; Yui, Noriko

Invent. Math. 99, No. 2, 225-246 (1990).

Newton’s binomial theorem is extended to an interesting non-commutative setting as follows: If, in a ring, \(ba=\gamma ab\) with \(\gamma\) commuting with a and b, then the (generalized) binomial coefficient \(\left( \begin{matrix} n\\ k\end{matrix} \right)_{\gamma}\) arising in the expansion \[ (a+b)^ n=\sum^{n}_{k=0}\left( \begin{matrix} n\\ k\end{matrix} \right)_{\gamma}a^{n-k}b^ k \] (resulting from these relations) is equal to the value at \(\gamma\) of the Gaussian polynomial \[ \left[ \begin{matrix} n\\ k\end{matrix} \right]=\frac{[n]}{[k][n-k]} \] where \([m]=(1- x^ m)(1-x^{m-1})...(1-x)\). (This is of course known in the case \(\gamma =1.)\)
From this it is deduced that in the (universal) \(C^*\)-algebra \(A_{\theta}\) generated by unitaries u and v such that \(vu=e^{2\pi i\theta}uv\), the spectrum of the self-adjoint element \((u+v)+(u+v)^*\) has all the gaps that have been predicted to exist, - provided that either \(\theta\) is rational, or \(\theta\) is a Liouville number. (In the latter case, the gaps are labelled in the natural way - via K-theory - by the set of all non-zero integers, and the spectrum is a Cantor set.)

Reviewer: G.A.Elliott

MSC:

46L55	Noncommutative dynamical systems
46L80	\(K\)-theory and operator algebras (including cyclic theory)

Keywords:

rotation algebra; Newton’s binomial theorem; non-commutative setting; Gaussian polynomial; Liouville number; spectrum; Cantor set

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