# American Institute of Mathematical Sciences

March  2013, 12(2): 881-898. doi: 10.3934/cpaa.2013.12.881

## A new class of $(H^k,1)$-rectifiable subsets of metric spaces

 1 CMAP, École Polytechnique -- Team GECO, INRIA Saclay, France 2 ENSTA ParisTech, UMA, and Team GECO, INRIA Saclay -- Île-de-France, Paris, France

Received  September 2011 Revised  June 2012 Published  September 2012

The main motivation of this paper arises from the study of Carnot--Carathéodory spaces, where the class of $1$-rectifiable sets does not contain smooth non-horizontal curves; therefore a new definition of rectifiable sets including non-horizontal curves is needed. This is why we introduce in any metric space a new class of curves, called continuously metric differentiable of degree $k$, which are Hölder but not Lipschitz continuous when $k>1$. Replacing Lipschitz curves by this kind of curves we define $(H^k,1)$-rectifiable sets and show a density result generalizing the corresponding one in Euclidean geometry. This theorem is a consequence of computations of Hausdorff measures along curves, for which we give an integral formula. In particular, we show that both spherical and usual Hausdorff measures along curves coincide with a class of dimensioned lengths and are related to an interpolation complexity, for which estimates have already been obtained in Carnot--Carathéodory spaces.
Citation: Roberta Ghezzi, Frédéric Jean. A new class of $(H^k,1)$-rectifiable subsets of metric spaces. Communications on Pure & Applied Analysis, 2013, 12 (2) : 881-898. doi: 10.3934/cpaa.2013.12.881
##### References:
 [1] A. Agrachev, D. Barilari and U. Boscain, On the Hausdorff volume in sub-Riemannian geometry,, Calc. Var. Partial Differential Equations, 43 (2012), 355. [2] L. Ambrosio and B. Kirchheim, Rectifiable sets in metric and Banach spaces,, Math. Ann., 318 (2000), 527. [3] L. Ambrosio and P. Tilli, "Topics on Analysis in Metric Spaces," volume 25 of Oxford Lecture Series in Mathematics and its Applications,, Oxford University Press, (2004). [4] P. Assouad, Plongements lipschitziens dans $R^n$, , Bull. Soc. Math. France, 111 (1983), 429. [5] A. Bellaïche, The tangent space in sub-Riemannian geometry,, In, (1996), 1. [6] A. S. Besicovitch, On the fundamental geometrical properties of linearly measurable plane sets of points,, Math. Ann., 98 (1928), 422. [7] U. Boscain, G. Charlot, R. Ghezzi and M. Sigalotti, Lipschitz classification of two-dimensional almost-Riemannian distances on compact oriented surfaces,, preprint, (2011). doi: 10.1007/s12220-011-9262-4. [8] E. Falbel and F. Jean, Measures of transverse paths in sub-Riemannian geometry,, J. Anal. Math., 91 (2003), 231. [9] H. Federer, The $(\varphi,k)$ rectifiable subsets of $n$-space,, Trans. Amer. Soc., 62 (1947), 114. [10] H. Federer, "Geometric Measure Theory,", Die Grundlehren der mathematischen Wissenschaften, (1969). [11] B. Franchi, R. Serapioni and F. Serra Cassano, Rectifiability and perimeter in the Heisenberg group,, Math. Ann., 321 (2001), 479. [12] J.-P. Gauthier, B. Jakubczyk and V. Zakalyukin, Motion planning and fastly oscillating controls,, SIAM J. Control Optim., 48 (): 3433. [13] J.-P. Gauthier and V. Zakalyukin, On the one-step-bracket-generating motion planning problem,, J. Dyn. Control Syst., 11 (2005), 215. [14] M. Gromov, Carnot-Carathéodory spaces seen from within,, In, (1996), 79. [15] G. H. Hardy, Weierstrass's non-differentiable function,, Trans. Amer. Math. Soc., 17 (1916), 301. [16] F. Jean, "Paths in Sub-Riemannian Geometry,", Springer (A. Isidori, (2000). [17] F. Jean, Entropy and complexity of a path in sub-Riemannian geometry,, ESAIM Control Optim. Calc. Var., 9 (2003), 485. [18] B. Kirchheim, Rectifiable metric spaces: local structure and regularity of the Hausdorff measure,, Proc. Amer. Math. Soc., 121 (1994), 113. [19] V. Magnani, Characteristic points, rectifiability and perimeter measure on stratified groups,, J. Eur. Math. Soc., 8 (2006), 585. [20] J. M. Marstrand, The $(\varphi, s)$ regular subsets of $n$-space,, Trans. Amer. Math. Soc., 113 (1964), 369. [21] P. Mattila, "Geometry of Sets and Measures in Euclidean Spaces: Fractals and Rectifiability,", Cambridge Studies in Advanced Mathematics. Cambridge University Press, (1996). [22] P. Mattila, R. Serapioni and F. Serra Cassano, Characterizations of intrinsic rectifiability in Heisenberg groups,, Ann. Sc. Norm. Super. Pisa Cl. Sci., 9 (2010), 687. [23] E. F. Moore, Density ratios and $(\phi,1)$ rectifiability in $n$-space,, Trans. Amer. Math. Soc., 69 (1950), 324. [24] D. Preiss, Geometry of measures in $R^n$: distribution, rectifiability, and densities,, Ann. of Math., 125 (1987), 537. [25] K. Weierstrass, On continuous functions of a real argument that do not have a well-defined differential quotient,, G.A. Edgar, (1993).

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##### References:
 [1] A. Agrachev, D. Barilari and U. Boscain, On the Hausdorff volume in sub-Riemannian geometry,, Calc. Var. Partial Differential Equations, 43 (2012), 355. [2] L. Ambrosio and B. Kirchheim, Rectifiable sets in metric and Banach spaces,, Math. Ann., 318 (2000), 527. [3] L. Ambrosio and P. Tilli, "Topics on Analysis in Metric Spaces," volume 25 of Oxford Lecture Series in Mathematics and its Applications,, Oxford University Press, (2004). [4] P. Assouad, Plongements lipschitziens dans $R^n$, , Bull. Soc. Math. France, 111 (1983), 429. [5] A. Bellaïche, The tangent space in sub-Riemannian geometry,, In, (1996), 1. [6] A. S. Besicovitch, On the fundamental geometrical properties of linearly measurable plane sets of points,, Math. Ann., 98 (1928), 422. [7] U. Boscain, G. Charlot, R. Ghezzi and M. Sigalotti, Lipschitz classification of two-dimensional almost-Riemannian distances on compact oriented surfaces,, preprint, (2011). doi: 10.1007/s12220-011-9262-4. [8] E. Falbel and F. Jean, Measures of transverse paths in sub-Riemannian geometry,, J. Anal. Math., 91 (2003), 231. [9] H. Federer, The $(\varphi,k)$ rectifiable subsets of $n$-space,, Trans. Amer. Soc., 62 (1947), 114. [10] H. Federer, "Geometric Measure Theory,", Die Grundlehren der mathematischen Wissenschaften, (1969). [11] B. Franchi, R. Serapioni and F. Serra Cassano, Rectifiability and perimeter in the Heisenberg group,, Math. Ann., 321 (2001), 479. [12] J.-P. Gauthier, B. Jakubczyk and V. Zakalyukin, Motion planning and fastly oscillating controls,, SIAM J. Control Optim., 48 (): 3433. [13] J.-P. Gauthier and V. Zakalyukin, On the one-step-bracket-generating motion planning problem,, J. Dyn. Control Syst., 11 (2005), 215. [14] M. Gromov, Carnot-Carathéodory spaces seen from within,, In, (1996), 79. [15] G. H. Hardy, Weierstrass's non-differentiable function,, Trans. Amer. Math. Soc., 17 (1916), 301. [16] F. Jean, "Paths in Sub-Riemannian Geometry,", Springer (A. Isidori, (2000). [17] F. Jean, Entropy and complexity of a path in sub-Riemannian geometry,, ESAIM Control Optim. Calc. Var., 9 (2003), 485. [18] B. Kirchheim, Rectifiable metric spaces: local structure and regularity of the Hausdorff measure,, Proc. Amer. Math. Soc., 121 (1994), 113. [19] V. Magnani, Characteristic points, rectifiability and perimeter measure on stratified groups,, J. Eur. Math. Soc., 8 (2006), 585. [20] J. M. Marstrand, The $(\varphi, s)$ regular subsets of $n$-space,, Trans. Amer. Math. Soc., 113 (1964), 369. [21] P. Mattila, "Geometry of Sets and Measures in Euclidean Spaces: Fractals and Rectifiability,", Cambridge Studies in Advanced Mathematics. Cambridge University Press, (1996). [22] P. Mattila, R. Serapioni and F. Serra Cassano, Characterizations of intrinsic rectifiability in Heisenberg groups,, Ann. Sc. Norm. Super. Pisa Cl. Sci., 9 (2010), 687. [23] E. F. Moore, Density ratios and $(\phi,1)$ rectifiability in $n$-space,, Trans. Amer. Math. Soc., 69 (1950), 324. [24] D. Preiss, Geometry of measures in $R^n$: distribution, rectifiability, and densities,, Ann. of Math., 125 (1987), 537. [25] K. Weierstrass, On continuous functions of a real argument that do not have a well-defined differential quotient,, G.A. Edgar, (1993).
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