On recurrence relations for Sobolev orthogonal polynomials WD Evans, LL Littlejohn, F Marcellan, C Markett, A Ronveaux SIAM journal on mathematical analysis 26 (2), 446-467, 1995 | 104 | 1995 |

Orthogonal polynomial solutions of linear ordinary differential equations WN Everitt, KH Kwon, LL Littlejohn, R Wellman Journal of computational and applied mathematics 133 (1-2), 85-109, 2001 | 100 | 2001 |

Sobolev orthogonal polynomials and second-order differential equations KH Kwon, LL Littlejohn Rocky Mountain Journal of Mathematics 28 (2), 547-594, 1998 | 98 | 1998 |

On the classification of differential equations having orthogonal polynomial solutions LL Littlejohn Annali di matematica pura ed applicata 138 (1), 35-53, 1984 | 92 | 1984 |

The orthogonality of the Laguerre polynomials for positive integers k KH Kwon, LL Littlejohn Annals of Numerical Mathematics 2, 289-303, 1995 | 89 | 1995 |

Jacobi–Stirling numbers, Jacobi polynomials, and the left-definite analysis of the classical Jacobi differential expression WN Everitt, KH Kwon, LL Littlejohn, R Wellman, GJ Yoon Journal of Computational and Applied Mathematics 208 (1), 29-56, 2007 | 84 | 2007 |

Classification of classical orthogonal polynomials KH Kwon, LL Littlejohn J. Korean Math. Soc 34 (4), 973-1008, 1997 | 79 | 1997 |

Legendre polynomials, Legendre–Stirling numbers, and the left-definite spectral analysis of the Legendre differential expression WN Everitt, LL Littlejohn, R Wellman Journal of computational and applied mathematics 148 (1), 213-238, 2002 | 78 | 2002 |

A combinatorial interpretation of the Legendre-Stirling numbers G Andrews, L Littlejohn Proceedings of the American Mathematical Society 137 (8), 2581-2590, 2009 | 64 | 2009 |

Orthogonal polynomials and spectral theory: a survey WN Everitt, LL Littlejohn Orthogonal polynomials and their applications, IMACS Annals on Computing and …, 1991 | 61 | 1991 |

The Legendre–Stirling numbers GE Andrews, W Gawronski, LL Littlejohn Discrete mathematics 311 (14), 1255-1272, 2011 | 59 | 2011 |

On the classification of differential equations having orthogonal polynomial solutions—II AM Krall, LL Littlejohn Annali di matematica pura ed applicata 149 (1), 77-102, 1987 | 50 | 1987 |

An application of a new theorem on orthogonal polynomials and differential equations LL Littlejohn Quaestiones Mathematicae 10 (1), 49-61, 1986 | 50 | 1986 |

A general left-definite theory for certain self-adjoint operators with applications to differential equations LL Littlejohn, R Wellman Journal of Differential Equations 181 (2), 280-339, 2002 | 49 | 2002 |

Differential operators and the Legendre type polynomials WN Everitt, LL Littlejohn | 46 | 1988 |

Orthogonal polynomial solutions to ordinary and partial differential equations LL Littlejohn Orthogonal Polynomials and their Applications: Proceedings of an …, 1988 | 45 | 1988 |

The Jacobi–Stirling numbers GE Andrews, ES Egge, W Gawronski, LL Littlejohn Journal of Combinatorial Theory, Series A 120 (1), 288-303, 2013 | 43 | 2013 |

Orthogonal polynomials and singular Sturm-Liouville systems, I LL Littlejohn, AM Krall The Rocky Mountain journal of mathematics 16 (3), 435-479, 1986 | 38 | 1986 |

Distributional solutions of the hypergeometric differential equation LL Littlejohn, RP Kanwal Journal of mathematical analysis and applications 122 (2), 325-345, 1987 | 37 | 1987 |

Characterizations of orthogonal polynomials satisfying differential equations KH Kwon, LL Littlejohn, BH Yoo SIAM Journal on Mathematical Analysis 25 (3), 976-990, 1994 | 35 | 1994 |