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The Fourier Series is the representation of continuous-time, periodic signals in terms of complex exponentials. The Dirichletconditions suggest that discontinuous signals may have a Fourier Series representation so long as there are a finite number ofdiscontinuities. This seems counter-intuitive, however, as complex exponentials are continuous functions. It does not seempossible to exactly reconstruct a discontinuous function from a set of continuous ones. In fact, it is not. However, it can beif we relax the condition of exactly and replace it with the idea of almost everywhere. This is to say that the reconstruction isexactly the same as the original signal except at a finite number of points. These points, not necessarily suprisingly, occur atthe points of discontinuities.

Introduction

The Fourier Series is the representation of continuous-time, periodic signals in terms of complex exponentials. The Dirichlet conditions suggest that discontinuous signals may have a Fourier Series representationso long as there are a finite number of discontinuities. This seems counter-intuitive, however, as complex exponentials are continuous functions. It does not seem possible to exactly reconstruct adiscontinuous function from a set of continuous ones. In fact, it is not. However, it can be if we relax the conditionof 'exactly' and replace it with the idea of 'almost everywhere'. This is to say that the reconstruction isexactly the same as the original signal except at a finite number of points. These points, not necessarily surprisingly,occur at the points of discontinuities.

History

In the late 1800s, many machines were built to calculate Fourier coefficients and re-synthesize:

f N t n N N c n ω 0 n t
Albert Michelson (an extraordinary experimental physicist) built a machine in 1898 that could compute c n up to n ± 79 , and he re-synthesized
f 79 t n 79 -79 c n ω 0 n t
The machine performed very well on all tests except thoseinvolving discontinuous functions . When a square wave, like that shown in [link] , was inputed into the machine, "wiggles" around the discontinuities appeared, and even as the numberof Fourier coefficients approached infinity, the wiggles never disappeared - these can be seen in the last plot in [link] . J. Willard Gibbs first explained this phenomenon in 1899, and therefore thesediscontinuous points are referred to as Gibbs Phenomenon .

Explanation

We begin this discussion by taking a signal with a finite number of discontinuities (like a square pulse ) and finding its Fourier Series representation. We thenattempt to reconstruct it from these Fourier coefficients. What we find is that the more coefficients we use, the morethe signal begins to resemble the original. However, around the discontinuities, we observe rippling that does not seem tosubside. As we consider even more coefficients, we notice that the ripples narrow, but do not shorten. As we approachan infinite number of coefficients, this rippling still does not go away. This is when we apply the idea of almosteverywhere. While these ripples remain (never dropping below 9% of the pulse height), the area inside them tends to zero,meaning that the energy of this ripple goes to zero. This means that their width is approaching zero and we can assertthat the reconstruction is exactly the original except at the points of discontinuity. Since the Dirichlet conditionsassert that there may only be a finite number of discontinuities, we can conclude that the principle of almosteverywhere is met. This phenomenon is a specific case of nonuniform convergence .

Questions & Answers

Do somebody tell me a best nano engineering book for beginners?
s. Reply
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Devang Reply
are you nano engineer ?
s.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
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
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SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
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Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
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Cied
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what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
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Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
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Porter
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Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
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Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
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Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Signals and systems. OpenStax CNX. Aug 14, 2014 Download for free at http://legacy.cnx.org/content/col10064/1.15
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