Abstract. We prove inequalities of Hardy type for functions in Triebel-Lizorkin spaces F s pq (G) on a domain G ⊂ R n , whose boundary has the Aikawa dimension strictly less than n − sp.
IntroductionIn this paper, we study Hardy-type inequalities for functions in Triebel-Lizorkin spaces F s pq (G); see [26] for the case of bounded smooth domains G. We assume that the boundary ∂G of a domain G is 'thin', in the sense that its Aikawa dimension is strictly smaller than n − sp. The notion of the Aikawa dimension appears in connection with the quasiadditivity of Riesz capacity, [2]; subsequently, it has turned out to be useful in other questions in the theory of function spaces, see e.g. [10,23]. In particular, it is known that, for every f ∈ C ∞ 0 (G), a 'classical' Hardy inequalityholds if 1 < p < n and ∂G is 'thin', i.e., if dim A (∂G) < n − p. Indeed, as it is observed in [17], this result is implicitly contained in [16]. On the other hand, it is well known that inequality (1.1) holds if R n \ G is (1, p) uniformly fat with 1 < p ≤ n, we refer to [20]. These two last results exhibit a dichotomy between 'thin' and 'fat' sets which manifests in Hardy-type inequalities.Though our main result is Theorem 1.5, we also formulate and prove the following illustrative theorem under an additional assumption that G is a John domain.1.2. Theorem. Let n ≥ 2, 1 < p < ∞, and 0 < s < min{1, n/p}. Suppose that G is a Johnwhere a constant C depends on parameters n, s, p, and G.Recall that bounded Lipschitz domains, and bounded domains with the interior cone condition, are John domains. Also, the Koch snowflake G is a John domain with dim A (∂G) = log 4/ log 3. For bounded Lipschitz domains G in the 'fat' case of sp > 1, inequality (1.3) holds for every f ∈ C ∞ 0 (G) without the L p -term f L p (G) on the right hand side; furthermore, the L p -term cannot2000 Mathematics Subject Classification. 46E35, 26D15.