Figure 7. The left-hand picture illustrates the
points in the orbit of a set; the flower picture at
center left, under a Möbius transformation. The
picture at center right reveals that it is a “tiling”,
where the initial tile is shown on the right.
Mandelbrot caused many to look anew at natural
objects in geometrical terms. Figure from [2].
I am one of many whose life and career have
been influenced enormously by Benoît and his
work, both directly and indirectly. We miss him,
but the legacy of his ideas and work will remain
with us all and with those who follow.
Bruce J. West
The Intermittent Distribution of the Stars in
the Heavens
Benoît’s idiosyncratic method of communicat-
ing mathematical ideas was both challenging
and refreshing. The introduction of geometri-
cal and statistical fractals into the scientific
lexicon opened up a new way of viewing na-
ture for a generation of scientists and allowed
them to understand complexity and scaling in
everything from surface waves on the ocean to the
irregular beating of the heart to the sequencing
of DNA. This accelerated the early research done
by biologists, physicians, and physicists on the
understanding of complex phenomena.
The line between what was proven and what
was conjecture in Benoît’s work was often ob-
scure to me, but in spite of that, or maybe even
because of that lack of clarity, I was drawn into
discussions on how to apply the mathematics
of fractals to complex phenomena. Fractals be-
gan as descriptive measures of static objects,
but dynamic fractals were eventually used to de-
scribe complex dynamic phenomena that eluded
description by traditional differential equations.
Culturally, fractals formed the bridge between
the analytic functions of the nineteenth- and
twentieth-century physics of acoustics, diffusion,
wave propagation, and quantum mechanics to the
Bruce J. West is adjunct professor of physics at Duke Uni-
versity. His email address is bruce.j.west@us.army.mil.
twenty-first-century physics of anomalous diffu-
sion, fractional differential equations, fractional
stochastic equations, and complex networks.
Benoît identified some common features of
complex phenomena and gave them mathemati-
cal expression without relying on the underlying
mechanisms. I used this approach to extract the
general properties of physiological time series,
which eventually led to the formation of a new
field of medical investigation called Fractal Phys-
iology, the title of a book [28] I coauthored in
1995 and the subject of an award-winning book
[29] on the fractional calculus. Later, in 2010, I be-
came founding editor-in-chief of the new journal
Frontiers in Fractal Physiology, which recognizes
the importance of fractal concepts in human
physiology and medicine.
I first met Benoît when I was a graduate student
in physics at the University of Rochester. Elliott
Montroll, who had the Einstein Chair in Physics
and who had been a vice president for research
at IBM, was friends with Benoît and would invite
him to come and give physics colloquia. In the
late 1960s, before the birth of fractals, I heard
Benoît conjecture as to why the night sky was
not uniformly illuminated because of the inter-
mittent distribution of stars in the heavens, why
the price of corn did not move smoothly in the
market but changed erratically, and why the time
between messages on a telephone trunkline were
not Poisson distributed as everyone had assumed.
These problems and others like them struck me
as much more interesting than calculating per-
turbation expansions of a nuclear potential. So
I switched fields and became a postdoctoral re-
searcher in statistical physics with Elliott. I have
interacted with many remarkable scientists, and
Benoît is at the top of that list. I am quite sure
that my decision to change fields was based in
large part on Mandelbrot’s presentations and the
subsequent discussion with him and Montroll.
Marc-Olivier Coppens
Engineering Complexity By Applying
Recursive Rules
As a chemical engineering researcher who worked
with Benoît since the middle of the 1990s, I
benefited a lot from his mentorship. I also miss
him a lot as a friend. In 1996, while completing my
Ph.D. thesis, I worked closely with him for several
months at Yale, sharing an office with Michael
Frame. I developed, with Benoît, a new way to
Marc-Olivier Coppens is professor and associate direc-
tor of the Multiscale Science and Engineering Center,
Rensselaer Polytechnic Institute. His email address is
coppens@rpi.edu.
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Notices of the AMS
Volume
59, Number 9
Figure 8. A right-angle Sierpinski triangle. Benoît
realized that such objects were not freaks and
belonged in mainstream mathematics. Analysis
on
fractals is now a fascinating area of
mathematics.
generate multifractals by taking the product of
harmonics of periodically extended functions.
Fractals in chemical engineering have affected
the modeling and characterization of various
porous materials. As Mandelbrot liked to say
in later years, fractals are an ideal way to mea-
sure “roughness”, and roughness is prevalent in
chemical engineering and materials science. The
roughness of porous media affects transport and
reactions in them and hence has a significant im-
pact on chemical engineering. For example, in my
thesis I showed how molecular-scale roughness
of porous catalysts influences chemical product
distributions up to industrial scales.
In my research I have used fractal trees to
interpolate efficiently between the micro- and
the macroscale, as in nature. Scaling up from
the laboratory to the production scale requires
preservation of small-scale, controlled features
up to larger scales. This challenge is met by
distributing or collecting fluid in a uniform way,
as is realized by scaling fractal architectures in
nature, such as trees, lungs, kidneys, and the
vascular network. Specifically, I proposed a fractal,
treelike injector to uniformly distribute fluids over
a reactor volume, so that the fluids can mix and
interact with the reactor contents. This patented
fractal injector has proven very efficient for gas-
solid fluidized beds. My laboratory is currently
developing a fractal fuel cell design, inspired by
the structure of the lung.
Benoît has had a major influence on my think-
ing. To a large extent, thanks or due to the advance
Figure 9. An invariant measure on a fractal
attractor of a system of three similitudes has
here been rendered in shades of green. (Bright
green =
=
= greater “density”, black=least “density”.)
of massively parallel, high-performance comput-
ers, chemical, biological, and materials sciences
are increasingly atomistic, deconstructing and
constructing matter out of individual elements in
which the details of each component and its inter-
actions are more and more explicitly accounted
for. This atomistic treatment is very powerful and
facilitates the study of specific properties of mat-
ter. However, sometimes the importance of the
forest tends to be lost in looking too closely at one
tree. The complementary, holistic view is, in my
opinion, extremely powerful as well, as it allows us
to see essential features in a phenomenon without
the need to resolve every detail. Fractals are an
example of this idea, where complexity emerges
from the combination of simple rules. A marriage
between the holistic and atomistic views can lead
us beyond the deficiencies of each one separately.
Nathan Cohen
Complexity Was Well Modeled by Fractals
Mathematicians spar in an uncomfortable match
between the pure and applied, in which migration
from one to the other is one way, and no one is
allowed to do both. But Benoît Mandelbrot did.
My interest in fractals stems from needing to
solve real-world problems. In 1985 I was a newly
minted Ph.D. in Cambridge (MA). There the general
view was that fractals were a “flavor” of the month,
and they were treated as an a posteriori paradigm
with no evidence of solving problems unsolved
in other ways. But I read The Fractal Geometry
Nathan Cohen is the founder of Fractal Antenna Systems,
Waltham, Massachusetts. His email address is ncohen@
fractenna.com.
October
2012
Notices of the AMS
1215