# MATH 522: Topics in Real Analysis, Spring 2007

### Description:

Geometric Measure Theory treats measure-theoretic properties of geometrically defined sets of various dimensions.  Some of the critical notions are  Hausdorff measure, rectifiable sets, and rectifiable currents.  The k dimensional Hausdorff (outer) measure

H k(A) gives, for every nonnegative number  k ,  a precise notion of the  k  dimensional size of  A for any subset  A  of a metric space  X .  In case  X= R 3 ,   C is a smooth embedded curve,  S  is a smooth embedded surface, and  U  is an open region.  Then  H 1(C) is the length of  C , H2 (S)  is the area of  S ,  and  H 3(U)  is the volume or Lebesgue measure of  U .  Also  H2 (C)=H 2 (C)=H 3(S)=0   while  H1(S)=H 1(U) =H2(U)= oo.  Noninteger Hausdorff measures are useful for some fractals, e.g.
0 < Hlog2/log3(Cantor set) < oo .  For integer k , a subset  M  of  X is called   k rectifiable if   M   is  H k almost the countable union of Lipschitz images of subsets of  R k.   Even in a general metric space X , one may associate to H k  almost every point  a  in  M  a “tangent” k plane  Tan(M,a) .

By assigning, for  Hk  almost every point   a  in a k dimensional rectifiable set  M,  an integer multiplicity  m(a) and an orientation  v(a)  for  Tan(M,a) , one obtains a  k dimensional rectifiable chain T.  One may integrate smooth  k  forms of  R n  on  M .  The boundary of  T  is then given simply by  d T(f) = T(df)  for a  k-1  form  f , and the  mass of  T  is the is   J M m(a)dH ka .  The Compactness Theorem of Federer-Fleming for rectifiable currents implies the existence, for a given  k-1  dimensional rectifiable boundary, a  k  dimensional rectifiable current of minimum mass. This provides a solution of the “Plateau Problem” in general dimensions and in many metric space contexts.

These results have enjoyed new, more accessible proofs in the nineties based on properties of slices of sets or currents by general affine  n-k planes.  Also some of these notions have remarkably been carried over to more general metric spaces and to chains with coefficients in other groups.

We will try to present some of these new ideas in the course as well as many of the standard analytic and geometric constructions for rectifiable sets and currents.

Prerequesites for the course include some knowledge of basic analysis and measure theory as in Math 425, 426 and beginning topology as in Math 443.

### Meets:

We will meet initially MWF 10-10:50 in HB453 and discuss possible changes of time then.

### Some References:

F. Morgan, Geometric Measure Theory, A Beginner's Guide,   Academic Press, 1988.
H. Federer, Geometric Measure Theory, Springer-Verlag, 1970.
P. Mattilla, Geometry of Sets and Measures in Euclidean Spaces, Cambridge, 1995.
L. Simon, Lectures on geometric measure theory. Australian National University, 1983
B. White, A new proof of Federer's structure theorem for k-dimensional subsets of  RN. J.A.M.S. 11 (1998),693- 701.

B. Kirchheim, Rectifiable Metric Spaces, Local structure and Regularity of the Hausdorff Measures, Proc. A.M.S., 121 (1994)113-123.
L. Ambrosio & B. Kirchheim, Rectifiable sets in metric and Banach spaces. Math. Ann.318(2000), 527--555.
L. Ambrosio & B. KirchheimCurrents in metric spaces. Acta Math. 185 (2000), 1--80.

J. Heinenon, Lectures on analysis in Metric Spaces, Springer, 2000.

### Instructor:

Robert Hardt    Office: Herman Brown 430; Office hours: 11-12 MWF (and others by appt.),

Email: hardt@rice.edu, Telephone: ext 3280

### Homepage:

http://math.rice.edu/~hardt/522S07/