ABSTRACT. In this paper, the abstract Fatou-Naim-Doob theorem is used to investigate the boundary behavior of positive solutions of the heat equation on the semi-infinite slab X = Rn_1 x R+ x (0, X). The concept of semifine limit is introduced, and relationships are obtained between fine, semifine, parabolic, one-sided parabolic and two-sided parabolic limits at points on the parabolic boundary of X. A Carleson-Calderón-type local Fatou theorem is also obtained for solutions on a union of two-sided parabolic regions.
Introduction.The boundary behavior of positive solutions of second-order parabolic equations on a horizontal boundary has been studied in [10] by applying the abstract Fatou-Naim-Doob theorem (cf. [13]). The main aim of this paper is to apply the same methods to investigate the boundary behavior of positive solutions on a vertical boundary. This subject has already been studied by classical methods in [5-8, and 14].This paper obtains the Calderón-type result (Theorem A) in [14] and special cases of results in [7,8] by means of fine convergence. The advantage of using this method is that, although the results in this paper are for solutions of the heat equation, it is clear (cf. [10]) that if a suitable integral representation is obtained then the results in § §2, 3, 6, and 7 still hold for positive solutions of more general parabolic equations on the semi-infinite slab X = R™-1 x R+ x (0, T). In particular the Calderón-type local Fatou theorem with two-sided parabolic approach regions (Theorem 7.3) would still hold (cf. [5]).Although the main interest is in behavior at the vertical boundary, this paper does not consider only the right half-space, but rather the semi-infinite slab X, this obtaining results for both horizontal and vertical boundaries. To do this, new nonsemithin sets are obtained in §3 and new Harnack inequalities are found in §4.I would like to thank Professor J. C. Taylor of McGill University, Montréal, for his continuing interest in my work.1. Preliminaries. Throughout this paper, 0 < T < oo and n E N are fixed. X denotes the semi-infinite slab Rn_1 X R+ x (0,T) = {(x1,xn,t):x' E Rn_1,xn > 0,0 < t < T}, H = R""1 x R+ x {0}, V = Rnl x {0} x [0, T) and B = HU V.Then B is the parabolic boundary of X.