# Regular boundary point

A point on the boundary of a domain in a Euclidean space , , at which, for any continuous function on , the generalized solution of the Dirichlet problem in the sense of Wiener–Perron (see Perron method) takes the boundary value , that is,

The regular boundary points of form a set , at the points of which the complement is not a thin set; the set of irregular boundary points (cf. Irregular boundary point) is a polar set of type . If all points of are regular boundary points, then the domain is called regular with respect to the Dirichlet problem.

For to be a regular boundary point it is necessary and sufficient that in the intersection of with any neighbourhood of there is a superharmonic barrier (a function in such that , Lebesgue's criterion for a barrier). It was first shown by H. Lebesgue in 1912 that for the vertex of a sufficiently acute angle lying in need not be a regular boundary point.

Let

and let be the capacity of the set . For to be a regular boundary point it is necessary and sufficient that the series

diverges, or for , that the series

diverges, where

(Wiener's criterion).

For , a point is a regular boundary point if there is a continuous path , , such that , and for . When , a point is a regular boundary point if it can be reached by the vertex of a right circular cone belonging to in a sufficiently small neighbourhood of . In the case of a domain in the compactified space , , the point at infinity is always a regular boundary point; when , the point at infinity is a regular boundary point if there is a continuous path , , such that for , and .

#### References

[1] | M.V. Keldysh, "On the solvability and stability of the Dirichlet problem" Uspekhi Mat. Nauk , 8 (1941) pp. 171–232 (In Russian) |

[2] | N.S. Landkof, "Foundations of modern potential theory" , Springer (1972) (Translated from Russian) |

[3] | W.K. Hayman, P.B. Kennedy, "Subharmonic functions" , 1 , Acad. Press (1976) |

#### Comments

The polarity of the set of irregular boundary points is contained in the Kellogg–Evans theorem. See, e.g., [a1] for irregular boundary points in abstract potential theory.

#### References

[a1] | J. Bliedtner, W. Hansen, "Potential theory. An analytic and probabilistic approach to balayage" , Springer (1986) |

[a2] | H. Lebesgue, "Sur des cas d'impossibilité du problème de Dirichlet ordinaire" C.R. Séances Soc. Math. France , 41 (1913) pp. 17 |

[a3] | H. Lebesgue, "Conditions de régularité, conditions d'irrégularité, conditions d'impossibilité dans le problème de Dirichlet" C.R. Acad. Sci. Paris , 178 (1924) pp. 349–354 |

[a4] | N. Wiener, "The Dirichlet problem" J. Math. Phys. , 3 (1924) pp. 127–146 |

[a5] | M. Tsuji, "Potential theory in modern function theory" , Chelsea, reprint (1975) |

[a6] | J. Wermer, "Potential theory" , Lect. notes in math. , 408 , Springer (1981) |

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Regular boundary point.

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