Fractal string

Ordinary fractal strings

An ordinary fractal string ${\displaystyle \omega }$ is a bounded, open subset of the real number line. Any such subset can be written as an at-most-countable union of connected open intervals with associated lengths ${\displaystyle {\mathcal {L}}=\{\ell _{1},\ell _{2},\ldots \}}$ written in non-increasing order. We allow ${\displaystyle \omega }$ to consist of finitely many open intervals, in which case ${\displaystyle {\mathcal {L}}}$ consists of finitely many lengths. We refer to ${\displaystyle {\mathcal {L}}}$ as a fractal string.

Example

The middle third's Cantor set is constructed by removing the middle third from the unit interval ${\displaystyle (0,1)}$, then removing the middle thirds of the subsequent intervals, ad infinitum. The deleted intervals ${\displaystyle \Omega =\left\{\left({\frac {1}{3}},{\frac {2}{3}}\right),\left({\frac {1}{9}},{\frac {2}{9}}\right),\left({\frac {7}{9}},{\frac {8}{9}}\right),\ldots \right\}}$ have corresponding lengths ${\displaystyle {\mathcal {L}}=\left\{{\frac {1}{3}},{\frac {1}{9}},{\frac {1}{9}},\ldots \right\}}$. Inductively, we can show that there are ${\displaystyle 2^{n-1}}$ intervals corresponding to each length of ${\displaystyle 3^{-n}}$. Thus, we say that the multiplicity of the length ${\displaystyle 3^{-n}}$ is ${\displaystyle 2^{n-1}}$.

Heuristic

The geometric information of the Cantor set in the example above is contained in the ordinary fractal string ${\displaystyle {\mathcal {L}}}$. From this information we can compute the box-counting dimension of the Cantor set. This notion of fractal dimension can be generalized to that of complex dimension, which will give us complete geometrical information regarding the local oscillations in the geometry of the Cantor set.

The geometric zeta function

If ${\displaystyle \sum _{j\in \mathbb {J} }{\ell _{j}}<\infty ,}$ we say that ${\displaystyle \Omega }$ has a geometric realization in ${\displaystyle \mathbb {R} ,}$ ${\displaystyle \Omega =\bigcup _{i=1}^{\infty }I_{i}}$, where the ${\displaystyle I_{i}}$ are intervals in ${\displaystyle \mathbb {R} }$, of all the lengths ${\displaystyle \{\ell _{j}\}_{j\in \mathbb {J} }}$, taken with multiplicity.

For each fractal string ${\displaystyle {\mathcal {L}}}$, we can associate to ${\displaystyle {\mathcal {L}}}$ a geometric zeta function ${\displaystyle \zeta _{\mathcal {L}}}$ defined as the Dirichlet series ${\displaystyle \zeta _{\mathcal {L}}(s)=\sum _{j\in \mathbb {J} }\ell _{j}^{s}}$. Poles of the geometric zeta function ${\displaystyle \zeta _{\mathcal {L}}(s)}$ are called complex dimensions of the fractal string ${\displaystyle {\mathcal {L}}}$. The general philosophy of the theory of complex dimensions for fractal strings is that complex dimensions describe the intrinsic oscillation in the geometry, spectra and dynamics of the fractal string ${\displaystyle {\mathcal {L}}}$.

The abscissa of convergence of ${\displaystyle \zeta _{\mathcal {L}}(s)}$ is defined as ${\displaystyle \sigma =\inf \left\{\alpha \in \mathbb {R} :\sum _{j=1}^{\infty }\ell _{j}^{\alpha }<\infty \right\}}$.

For a fractal string ${\displaystyle {\mathcal {L}}}$ with infinitely many nonzero lengths, the abscissa of convergence ${\displaystyle \sigma }$ coincides with the Minkowski dimension of the boundary of the string, ${\displaystyle \partial \Omega }$. For our example, the boundary Cantor string is the Cantor set itself. So the abscissa of convergence of the geometric zeta function ${\displaystyle \zeta _{\mathcal {L}}(s)}$ is the Minkowski dimension of the Cantor set, which is ${\displaystyle {\frac {\log 2}{\log 3}}}$.

Complex dimensions

For a fractal string ${\displaystyle {\mathcal {L}}}$, composed of an infinite sequence of lengths, the complex dimensions of the fractal string are the poles of the analytic continuation of the geometric zeta function associated with the fractal string. (When the analytic continuation of a geometric zeta function is not defined to all of the complex plane, we take a subset of the complex plane called the "window", and look for the "visible" complex dimensions that exist within that window.[1])

Example

Continuing with the example of the fractal string associated to the middle thirds Cantor set, we compute ${\displaystyle \zeta _{\mathbb {C} }(s)=\sum _{n=1}^{\infty }{\frac {2^{n-1}}{3^{ns}}}={\frac {\frac {1}{3^{s}}}{1-{\frac {2}{3^{s}}}}}}$. We compute the abscissa of convergence to be the value of ${\displaystyle s}$ satisfying ${\displaystyle 3^{s}=2}$, so that ${\displaystyle s=\log _{3}2={\frac {\log 2}{\log 3}}}$ is the Minkowski dimension of the Cantor set.

For complex ${\displaystyle s}$, ${\displaystyle \zeta _{\mathbb {C} }(s)}$ has poles at the infinitely many solutions of ${\displaystyle 3^{s}=2}$, which, for this example, occur at ${\displaystyle s={\frac {\log 2+2\pi ik}{\log 3}}}$, for all integers ${\displaystyle k}$. This collection of points is called the set of complex dimensions of the middle thirds Cantor set.

Applications

For fractal strings associated with sets like Cantor sets, formed from deleted intervals that are rational powers of a fundamental length, the complex dimensions appear in a regular, arithmetic progression parallel to the imaginary axis, and are called lattice fractal strings. Sets that do not have this property are called non-lattice. There is a dichotomy in the theory of measures of such objects: an ordinary fractal string is Minkowski measurable if and only if it is non-lattice.

In,[1] the existence of non-real complex dimensions with positive real part are proposed to be the signature feature of fractal objects. Formally in,[1] Michel Lapidus and Machiel van Frankenhuijsen propose to define “fractality” as the presence of at least one nonreal complex dimension with positive real part. This new definition of fractality solves some old problems in Fractal Geometry. For example, everyone can agree that the Cantor's Devil Staircase is fractal, which it is with this new definition of fractality in terms of complex dimensions, but it is not in the sense of Mandelbrot.

A Spectral Reformulation of the Riemann Hypothesis by Michel Lapidus and Helmut Maier

Following the examples of the a-string and of the ordinary Cantor string given by Michel Lapidus in the 1990s, the notion of a fractal string was conceived and defined by Michel Lapidus and Carl Pomerance in their investigation of the one-dimensional Weyl-Berry conjecture for fractal drums and its connection with the Riemann zeta function ~\cite{LapPo}. The Riemann hypothesis turned out to be equivalent to the solvability of the corresponding inverse spectral problem for fractal strings, as was established by Michel Lapidus and Helmut Maier in ~\cite{LapMa}. The heuristic notion of complex dimensions then started to emerge and was used in a crucial way in their spectral reformulation of the Riemann hypothesis. The precise notion of complex dimensions, defined as the poles of the geometric zeta function associated with the fractal string, was crystallized and rigorously developed by Michel Lapidus and Machiel van Frankenhuijsen in the research monograph Fractal Geometry and Number Theory \cite{L-vF1} and then significantly further extended in the book Fractal Geometry, Complex Dimensions and Zeta Functions ~\cite{L-vF2}.

The work of Michel Lapidus and Helmut Maier can be summarized as follows:

The inverse spectral problem for a fractal string can be solved if and only if its dimension is not 1/2.

The inverse spectral problem for a fractal string is not solvable in dimension 1/2 because the Riemann zeta function $\zeta(s)=1+1/2^s+1/3^s+\cdots$ vanishes (infinitely often) on the critical line $\Re(s)=1/2$. Therefore, the inverse spectral problem is solvable in dimension $D\neq 1/2$ if and only if the Riemann zeta function does not vanish off the critical line $\Re(s)=1/2$ or, equivalently, if and only if the Riemann hypothesis is true.

References

1. ^ a b c M. L. Lapidus, M. van Frankenhuijsen, Fractal Geometry, Complex Dimensions and Zeta Functions: Geometry and Spectra of Fractal Strings, Monographs in Mathematics, Springer, New York, Second revised and enlarged edition, 2012. doi:10.1007/978-1-4614-2176-4