Summary. In their classic paper, S. Orey and S.J. Taylor compute the Hausdorff dimension of the set of points at which the law of the iterated logarithm fails for Brownian motion. By introducing "fast sets", we describe a converse to this problem for fractional Brownian motion. Our result is in the form of a limit theorem. From this, we can deduce refinements to the aforementioned dimension result of Orey and Taylor as well as the work of R. Kaufman. This is achieved via establishing relations between stochastic co-dimension of a set and its Hausdorff dimension along the lines suggested by a theorem of S.J. Taylor. Suppose W , W (t); t 0 is standard one-dimensional Brownian motion starting at 0. Continuity properties of the process W form a large part of classical probability theory. In particular, we mention A. Khintchine's law of the iterated logarithm (see, for example, [21, Theorem II.1.9]): for each t 0, there exists a null set N 1 (t) such that for all ω ∈ N 1 (t), lim sup
Keywords andLater on, P. Lévy showed that ∪ t 0 N 1 (t) is not a null set. Indeed, he showed the existence of a null set N 2 outside which lim sup The main result of [18] is that with probability one,One can think of this as the multi-fractal analysis of white noise. Above and throughout, "dim(A)" refers to the Hausdorff dimension of A. Furthermore, whenever dim(A) is (strictly) negative, we really mean A = ?. Orey and Taylor's discovery of Eq. (1.3) relied on special properties of Brownian motion. In particular, they used the strong Markov property in an essential way. This approach has been refined in [3,4,11], in order to extend (1.3) in several different directions.Our goal is to provide an alternative proof of Eq. (1.3) which is robust enough to apply to non-Markovian situations. We will do so by (i) viewing F 1 (λ) as a random set and considering its hitting probabilities; and (ii) establishing (within these proofs) links between Eqs. (1.2) and (1.3).To keep from generalities, we restrict our attention to fractional Brownian motion. With this in mind, let us fix some α ∈ ]0, 2[ and define X , X(t); t 0 to be a one-dimensional Gaussian process with stationary increments, mean zero and incremental standard deviation given by,See (1.8) for our notation on L p (P) norms. The process X is called fractional Brownian motion with index α -hereforth written as f BM(α). We point out that when α = 1, X is Brownian motion.
Let dim M (E) denote the upper Minkowski dimension of a Borel set E ⊂ R1 ; see references [17,24]. Our first result, which is a fractal analogue of Eq. (1.2), is the following limit theorem: Theorem 1.1. Suppose X is f BM(α) and E ⊂ [0, 1] is closed. With probability one,(
1.4)On the other hand, with probability one,(1.5) SÉMINAIRE DE PROBABILITÉS XXXIV, Lec. Notes in Math. 393-416 (2000) Loosely speaking, when α = 1, Theorem 1.1 is a converse to (1.3). For all λ 0, define F α (λ) to be the collection of all closed sets E ⊂ [0, 1] such that lim supOne can think of the elements of F α (λ) as λ-fast sets. Theorem 1.1 can be...