Sunday, 16 September 2012

The Geometry of Recurrent Families of Polynomials

The Chebyshev T-polynomials have at least two natural definitions, either via the characterizing property cos(ntheta)=Tn(cos(theta)), which I will call the geometric definition, or via a recurrence relation Tn+1=2xTnTn1. My question concerns the relationship between these two definitions, and specifically asks if other families of polynomials defined by similar recurrence relations have a natural geometric interpretation, similarly to how Tn expresses the relation between x-coordinates of particular points on the circle.



Starting from the geometric definition of Tn, it is straightforward to derive the recurrent definition. Is there a natural way of going the other direction? One thought I have had is as follows. If we treat the Tn as elements of the coordinate ring of the circle, then Tn expresses the relationship between the two parameterizations of the x-coordinates given by thetamapstocos(theta) and thetamapstocos(ntheta). Can we do the same sort of thing with other families of polynomials defined by similar recurrence relations (say, for simplicity, second-order linear polynomial recurrences with coefficients of degree leq1)?



One potential obstacle I have encountered is that cosh(ntheta)=Tn(cosh(theta)) as well, so that the hyperbola X2Y2=1 is just as natural a choice as the circle for a geometric object associated to Tn.



Here's my main question: given a particular family of polynomials Pn related by a polynomial recurrence of an appropriately "nice" type, can one associate one or more algebraic curves (or other geometric objects) so that Pn expresses some relationship between various points on the curve?

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