# 0.11 Decision trees  (Page 5/5)

 Page 5 / 5

The dyadic decision trees studied here are different than classical tree rules, such as CART orC4.5. Those techniques select a tree according to

$\stackrel{^}{k}=arg\underset{k\ge 1}{min}\left\{{\stackrel{^}{R}}_{n},\left({\stackrel{^}{f}}_{n}^{\left(k\right)}\right),+,\alpha ,k\right\},$

for some $\alpha >0$ whereas ours was roughly

$\stackrel{^}{k}=arg\underset{k\ge 1}{min}\left\{{\stackrel{^}{R}}_{n},\left({\stackrel{^}{f}}_{n}^{\left(k\right)}\right),+,\alpha ,\sqrt{k}\right\},$

for $\alpha \approx \sqrt{\frac{3log2}{2n}}$ . The square root penalty is essential for the risk bound. No such bound exists for CARTor C4.5. Moreover, recent experimental work has shown that the square root penalty often performs better in practice. Finally,recent results show that a slightly tighter bounding procedure for the estimation error can be used to show thatdyadic decision trees (with a slightly different pruning procedure) achieve a rate of

$E\left[R\left({\stackrel{^}{f}}_{n}^{T}\right)\right]-{R}^{*}\phantom{\rule{4pt}{0ex}}=\phantom{\rule{4pt}{0ex}}O\left({n}^{-1/2}\right),\phantom{\rule{4pt}{0ex}}\phantom{\rule{4.pt}{0ex}}\text{as}\phantom{\rule{4.pt}{0ex}}n\to \infty ,$

which turns out to be the minimax optimal rate (i.e., under the boundary assumptions above, no method can achieve a faster rate of convergence tothe Bayes error).

## Box counting dimension

The notion of dimension of a sets arises in many aspects of mathematics, and it is particularly relevant to the study offractals (that besides some important applications make really cool t-shirts). The dimension somehow indicates how we shouldmeasure the contents of a set (length, area, volume, etc...). The box-counting dimension is a simple definition of the dimension ofa set. The main idea is to cover the set with boxes with sidelength $r$ . Let $N\left(r\right)$ denote the smallest number of such boxes, then the box counting dimension is defined as

$\underset{r\to 0}{lim}\frac{logN\left(r\right)}{-logr}.$

Although the boxes considered above do not need to be aligned on a rectangulargrid (and can in fact overlap) we can usually consider them over a grid and obtain an upper bound on the box-counting dimension. Toillustrate the main ideas let's consider a simple example, and connect it to the classification scenario considered before.

Let $f:\left[0,1\right]\to \left[0,1\right]$ be a Lipschitz function, with Lipschitz constant $L$ ( i.e., $|f\left(a\right)-f\left(b\right)|\le L|a-b|,\phantom{\rule{4pt}{0ex}}\forall a,b\in \left[0,1\right]$ ). Define the set

$A=\left\{x=\left({x}_{1},{x}_{2}\right):{x}_{2}=f\left({x}_{1}\right)\right\},$

that is, the set $A$ is the graphic of function $f$ .

Consider a partition with ${k}^{2}$ squared boxes (just like the ones we used in the histograms), the points in set $A$ intersect at most ${C}^{\text{'}}k$ boxes, with ${C}^{\text{'}}=\left(1+⌈L⌉\right)$ (and also the number of intersected boxes is greater than $k$ ). The sidelength of the boxes is $1/k$ therefore the box-counting dimension of $A$ satisfies

$\begin{array}{ccc}\hfill {dim}_{B}\left(A\right)& \le & \underset{1/k\to 0}{lim}\frac{log{C}^{\text{'}}k}{-log\left(1/k\right)}\hfill \\ & =& \underset{k\to \infty }{lim}\frac{log{C}^{\text{'}}+log\left(k\right)}{log\left(k\right)}\hfill \\ & =& 1.\hfill \end{array}$

The result above will hold for any “normal” set $A\subseteq {\left[0,1\right]}^{2}$ that does not occupy any area. For most sets the box-counting dimension is always going to be an integer, butfor some “weird” sets (called fractal sets) it is not an integer. For example, the Koch curvehas box-counting dimension $log\left(4\right)/log\left(3\right)=1.26186...$ . This means that it is not quite as small as a 1-dimensional curve, but not as big as a2-dimensional set (hence occupies no area).

To connect these concepts to our classification scenario consider a simple example. Let $\eta \left(x\right)=P\left(Y=1|X=x\right)$ and assume $\eta \left(x\right)$ has the form

$\eta \left(x\right)=\frac{1}{2}+{x}_{2}-f\left({x}_{1}\right),\phantom{\rule{1.em}{0ex}}\forall x\equiv \left({x}_{1},{x}_{2}\right)\in \mathcal{X},$

where $f:\left[0,1\right]\to \left[0,1\right]$ is Lipschitz with Lipschitz constant $L$ . The Bayes classifier is then given by

${f}^{*}\left(x\right)={\mathbf{1}}_{\left\{\eta \left(x\right)\ge 1/2\right\}}\equiv {\mathbf{1}}_{\left\{{x}_{2}\ge f\left({x}_{1}\right)\right\}}.$

This is depicted in [link] . Note that this is a special, restricted class of problems. That is, we areconsidering the subset of all classification problems such that the joint distribution ${P}_{XY}$ satisfies $P\left(Y=1|X=x\right)=1/2+{x}_{2}-f\left({x}_{1}\right)$ for some function $f$ that is Lipschitz. The Bayes decision boundary is therefore given by

$A=\left\{x=\left({x}_{1},{x}_{2}\right):{x}_{2}=f\left({x}_{1}\right)\right\}.$

Has we observed before this set has box-counting dimension 1.

Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
what does nano mean?
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
for screen printed electrodes ?
SUYASH
What is lattice structure?
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
how did you get the value of 2000N.What calculations are needed to arrive at it
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!