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The Sobolev-Laguerre polynomials form an orthogonal polynomial system with respect to a Sobolev-type inner product associated with the Laguerre measure on the positive half-axis and two point masses $M,N > 0$ at the origin involving functions and derivatives. These polynomials have attracted much interest over the last two decades, since they became known to satisfy, for any value of the Laguerre parameter $\alpha\in\mathbb{N}_{0}$, a spectral differential equation of finite order $4\alpha+10$. In this paper we establish a new explicit representation of the corresponding differential operator which consists of a number of elementary components depending on $\alpha,M,N$. Their interaction reveals a rich structure both being useful for applications and as a model for further investigations in the field. In particular, the Sobolev-Laguerre differential operator is shown to be symmetric with respect to the inner product., Comment: 22 pages

In the enduring, fruitful research on spectral differential equations with polynomial eigenfunctions, Koornwinder's generalized Laguerre polynomials are playing a prominent role. Being orthogonal on the positive half-line with respect to the Laguerre weight and an additional point mass $N \ge 0$ at the origin, these polynomials satisfy, for any $\alpha\in\mathbb{N}_{0}$, a linear differential equation of order $2\alpha+4$. In the present paper we establish a new elementary representation of the corresponding 'Laguerre-type' differential operator and show its symmetry with respect to the underlying weighted scalar product. Furthermore, we discuss various other representations of the operator, mainly given in factorized form, and show their equivalence. Finally, by applying a limiting process to the Laguerre-type equation, we deduce new elementary representations for the higher-order differential equation satisfied by the Bessel-type functions on the positive half-line., Comment: 16 pages

We investigate the differential equation for the Jacobi-type polynomials which are orthogonal on the interval $[-1,1]$ with respect to the classical Jacobi measure and an additional point mass at one endpoint. This scale of higher-order equations was introduced by J. and R. Koekoek in 1999 essentially by using special function methods. In this paper, a completely elementary representation of the Jacobi-type differential operator of any even order is given. This enables us to trace the orthogonality relation of the Jacobi-type polynomials back to their differential equation. Moreover, we establish a new factorization of the Jacobi-type operator which gives rise to a recurrence relation with respect to the order of the equation., Comment: 17 pages

For a long time it has been a challenging goal to identify all orthogonal polynomial systems that occur as eigenfunctions of a linear differential equation. One of the widest classes of such eigenfunctions known so far, is given by Koornwinder's generalized Jacobi polynomials with four parameters $\alpha,\beta\in\mathbb{N}_{0}$ and $M,N \ge 0$ determining the orthogonality measure on the interval $-1 \le x \le 1$. The corresponding differential equation of order $2\alpha+2\beta+6$ is presented here as a linear combination of four elementary components which make the corresponding differential operator widely accessible for applications. In particular, we show that this operator is symmetric with respect to the underlying scalar product and thus verify the orthogonality of the eigenfunctions., Comment: 11 pages