For general varifolds in Euclidean space, we prove an isoperimetric inequality, adapt the basic theory of generalised weakly differentiable functions, and obtain several Sobolev type inequalities. We thereby intend to facilitate the use of varifold theory in the study of diffused surfaces.MSC-classes 2010. 53A07 (Primary); 46E35, 49Q15 (Secondary).Keywords. Varifold, isoperimetric inequality, generalised weakly differentiable function, Sobolev inequality.
IntroductionGeneral aim. The isoperimetric inequality is well established in the context of sharp surfaces (e.g., integral currents, sets, or integral varifolds) in Euclidean space, but little appears to be known for diffused surfaces (i.e., for surfaces that are not concentrated on a set of the their own dimension). General varifolds form a very flexible model for the latter case; in fact, for equations of Allen-Cahn type, their utility was established by Ilmanen, Padilla, and Tonegawa (see [6] and [12]) and, for discrete and computational geometry, their unifying use has been recently suggested by Buet, Leonardi, and Masnou (see [3]). The present paper shall contribute to this proposed development by adapting several core tools to the possibly non-rectifiable case. To outline these results, suppose m and n are positive integers, m ≤ n, V is an m dimensional varifold in R n , and, to avoid case distinctions, also m > 1; see Section 2 for the notation.Isoperimetric inequality, see Section 3. The best result up to now (see the second author [13, 6.11]) did apply to general varifolds, but controlled only their rectifiable parts:where Γ is a positive, finite number determined by m. Following the first author (see [7, 2.2]), it unified the approach of Allard in [2, 7.1] and Michael and Simon in [11, 2.1]. Clearly, if 0 < d < ∞, and Θ m ( V , x) ≥ d for V almost all x, the result impliesWe notice that δV encodes both, the total mass of the variational boundary and the integral of the modulus of the generalised mean curvature of the varifold, see Allard [2, 4.3]; in particular, a more classical form results for varifolds with vanishing mean curvature (i.e., generalised minimal surfaces) and, by Allard [2, 4.8 (4)], the isoperimetric inequality for integral currents with nonoptimal constant is a special case. In 3.5 and 3.7, we establish that, if V (R n ) < ∞, thenBy homogeneity considerations, one may not replace (A(d), d −1/m ) by (R n , 1). The sets A(d), for suitable d, naturally describe the region, where the behaviour of the diffused surface resembles the behaviour of an m dimensional sharp surface.Generalised weakly differentiable functions, see Section 4. We extend the basic theory of generalised weakly differentiable functions (see the first author [9, § § 8-9] and [10, 4.1, 2]) from rectifiable varifolds to general varifolds. This theory includes the study of closedness properties (under convergence, composition, addition, and multiplication) and a coarea formula in functional analytic form. The main differences lie in the possible non-existence of decomposit...