We show that if a Barlow-Evans Markov process on a vermiculated space is
symmetric, then one can study the spectral properties of the corresponding
Laplacian using projective limits. For some examples, such as the Laakso spaces
and a Spierpinski P\^ate \`a Choux, one can develop a complete spectral theory,
including the eigenfunction expansions that are analogous to Fourier series.
Also, one can construct connected fractal spaces isospectral to the fractal
strings of Lapidus and van Frankenhuijsen.
We investigate the relation between simple random walks on repeated
barycentric subdivisions of a triangle and a self-similar fractal, Strichartz
hexacarpet, which we introduce. We explore a graph approximation to the
hexacarpet in order to establish a graph isomorphism between the hexacarpet
approximations and Barycentric subdivisions of the triangle, and discuss
various numerical calculations performed on the these graphs. We prove that
equilateral barycentric subdivisions converge to a self-similar geodesic metric
space of dimension log(6)/log(2), or about 2.58.
We study derivations and Fredholm modules on metric spaces with a local
regular conservative Dirichlet form. In particular, on finitely ramified
fractals, we show that there is a non-trivial Fredholm module if and only if
the fractal is not a tree (i.e. not simply connected). This result relates
Fredholm modules and topology, and refines and improves known results on p.c.f.
fractals.
A fractafold, a space that is locally modeled on a specified fractal, is the
fractal equivalent of a manifold. For compact fractafolds based on the
Sierpinski gasket, it was shown by the first author how to compute the discrete
spectrum of the Laplacian in terms of the spectrum of a finite graph Laplacian.
A similar problem was solved by the second author for the case of infinite
blowups of a Sierpinski gasket, where spectrum is pure point of infinite
multiplicity. Both works used the method of spectral decimations to obtain
explicit description of the eigenvalues and eigenfunctions.
We provide two methods for constructing smooth bump functions and for
smoothly cutting off smooth functions on fractals, one using a probabilistic
approach and sub-Gaussian estimates for the heat operator, and the other using
the analytic theory for p.c.f. fractals and a fixed point argument. The heat
semigroup (probabilistic) method is applicable to a more general class of
metric measure spaces with Laplacian, including certain infinitely ramified
fractals, however the cut off technique involves some loss in smoothness. From
the analytic approach we establish a Borel theorem for p.c.f.