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But to compute K – to compute the kernel function, all you need is order N time, because the kernel function is defined as X transpose Z squared, so you just take the inner product between X and Z, which is order N time and you square that and you’ve computed this kernel function, and so you just computed the inner product between two vectors where each vector has N squared elements, but you did it in N square time.

Student: For any kernel you find for X and Z, does Phi exist for X and Z?

Instructor (Andrew Ng) :Let me talk about that later. We’ll talk about what is a valid kernel later. Please raise your hand if this makes sense. So let me just describe a couple of quick generalizations to this. One is that if you define KXZ to be equal to X transpose Z plus C squared, so again, you can compute this kernel in order N time, then that turns out to correspond to a feature vector where I’m just going to add a few more elements at the bottom where you add root 2. Let me read that. That was root 2 CX1 root 2 CX2 root 2 CX3 and C.

And so this is a way of creating a feature vector with both the monomials, meaning the first order terms, as well as the quadratic or the inner product terms between XI and XJ, and the parameter C here allows you to control the relative waiting between the monomial terms, so the first order terms, and the quadratic terms. Again, this is still inner product between vectors of length and square [inaudible] in order N time.

More generally, here are some other examples of kernels. Actually, a generalization of the one I just derived right now would be the following kernel. And so this corresponds to using all N plus DQZ features of all monomials. Monomials just mean the products of XI XJ XK. Just all the polynomial terms up to degree D and plus [inaudible] so on the order of N plus D to the power of D, so this grows exponentially in D.

This is a very high dimensional feature vector, but again, you can implicitly construct the feature vector and take inner products between them. It’s very computationally efficient, because you just compute the inner product between X and Z, add C and you take that real number to the power of D and by plugging this in as a kernel, you’re implicitly working in an extremely high dimensional computing space.

So what I’ve given is just a few specific examples of how to create kernels. I want to go over just a few specific examples of kernels. So let’s you ask you more generally if you’re faced with a new machine-learning problem, how do you come up with a kernel? There are many ways to think about it, but here’s one intuition that’s sort of useful. So given a set of attributes of X, you’re going to use a feature vector of Phi of X and given a set of attributes Z, you’re going to use an input feature vector Phi of Z, and so the kernel is computing the inner product between Phi of X and Phi of Z.

And so one intuition – this is a partial intuition. This isn’t as rigorous intuition that it is used for. It is that if X and Z are very similar, then Phi of X and Phi of Z will be pointing in the same direction, and therefore the inner product would be large. Whereas in contrast, if X and Z are very dissimilar, then Phi of X and Phi of Z may be pointing different directions, and so the inner product may be small. That intuition is not a rigorous one, but it’s sort of a useful one to think about.

Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
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.
what is the actual application of fullerenes nowadays?
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.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Mueller Reply
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Source:  OpenStax, Machine learning. OpenStax CNX. Oct 14, 2013 Download for free at http://cnx.org/content/col11500/1.4
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