Applied left-definite theory : the Jacobi polynomials, their Sobolev orthogonality, and self-adjoint operators.

It is well known that, for –α, –β, –α – β – 1 ∉ ℕ, the Jacobi polynomials {Pn(α,β)(x)} ∞ n=0 are orthogonal on ℝ with respect to a bilinear form of the type(f,g)μ = ∫ℝfgdμ, for some measure μ. However, for negative integer parameters α and β, an application of Favard's theorem shows that the Jacobi polynomials cannot be orthogonal on the real line with respect to a bilinear form of this type for any positive or signed measure. But it is known that they are orthogonal with respect to a Sobolev inner product. In this work, we first consider the special case where α = β = –1. We shall discuss the Sobolev orthogonality of the Jacobi polynomials and construct a self-adjoint operator in a certain Hilbert-Sobolev space having the entire sequence of Jacobi polynomials as eigenfunctions. The key to this construction is the left-definite theory associated with the Jacobi differential equation, and the left-definite spaces and operators will be constructed explicitly. The results will then be generalized to the case where α > –1, β = –1.