Open access peer-reviewed chapter - ONLINE FIRST

# On the Zap Integral Operators over Fourier Transforms

By Juan Manuel Velazquez Arcos, Ricardo Teodoro Paez Hernandez, Alejandro Perez Ricardez, Jaime Granados Samaniego and Alicia Cid Reborido

Submitted: June 12th 2020Reviewed: October 21st 2020Published: November 5th 2020

DOI: 10.5772/intechopen.94573

## Abstract

We devote the current chapter to describe a class of integral operators with properties equivalent to a killer operator of the quantum mechanics theory acting over a determined state, literally killing the state but now operating over some kind of Fourier integral transforms that satisfies a certain Fredholm integral equation, we call this operators Zap Integral Operators (ZIO). The result of this action is to eliminate the inhomogeneous term and recover a homogeneous integral equation. We show that thanks to this class of operators we can explain the presence of two extremely different solutions of the same Generalized Inhomogeneous Fredholm equation. So we can regard the Generalized Inhomogeneous Fredholm Equation as a Super-Equation with two kinds of solutions, the resonant and the conventional but coexisting simultaneously. Also, we remember the generalized projection operators and we show they are the precursors of the ZIO. We present simultaneous academic examples for both kinds of solutions.

### Keywords

• integral operators
• generalized inhomogeneous Fredholm equations
• killer operators
• evanescent waves
• electromagnetic resonances

## 1. Introduction

Recently a new question about the solutions of integral Fredholm emerges, that is the question about the type of equation each of them solve. If we follow the steps or the clue marked by the linear second order differential equations the solutions of the inhomogeneous equation do not solve de homogeneous equation. But we have shown in a recent paper that both kind of solutions of the homogeneous and also the inhomogeneous Fredholm equations satisfy a third class of integral equation we named the Generalized Inhomogeneous Fredholm Equation (GIFE) which is only a bit different for the traditional inhomogeneous [1, 2, 3]. Even more, we can transform his appearance in a continuous form from homogeneous to inhomogeneous, but preserving his very extraordinary property: the two kinds of solutions are simultaneous solutions. This situation is quite different from differential equations but not the connection between eigenfunctions and solutions of inhomogeneous equations through the Green function [4, 5, 6, 7]. And if we want to explain this behavior we find a founder: an integral operator which is hidden in the structure of the GIFE. There is no surprise in the fact that the new operator treats in different manner both kinds of solutions. Indeed, it seems to be natural that the new operators include the Green function and are close to the Fredholm operator [2, 3]. Before we define the ZIO operators we must underline the fact that in a broadcasting situation [8, 9, 10] we must take into accounts not only one kind of traveling waves but all the known ones because the complete description of the phenomena comes from the GIFE. Another important goal of this paper is to give an explanation of the simultaneous validity of two sets of boundary conditions that are very apart one to the other and the fact that there is a connection with other projection operators, the generalized projection operators (GPO) [11] that separates the constituents of a signal in orthogonal parts.

## 2. Remembering the GIFE

We remember that if we take the inhomogeneous vector integral Fredholm Eq. (1):

umrω=umrω+λω0Knmωrr'unr'ωdr'E1

Where the kernel Knmωrr', is the product of the interaction Atmωrs(may be a non-local potential) with the free Green functionGntωrs.

And we make the ansatz of two successive approximations (a second order approach) [9], by the consideration that λωis a number with a very small absolute value (λω1), we arrive to the integral equation we named the GIFE:

smrω=smrω+Θmrω+νω0Knmωrr'snr'ωdr'E2

This last equation is the one that have the property of represent a complete panorama in a broadcasting problem, that is describes both the resonant and the conventional behavior of the electromagnetic field [12].

As we have commented, Eq. (2) carries a mechanism that allows simultaneously consider both types of solution. The so called generalized source is indeed a blend of integral operators as we will see with properties we want to visualize. But first we must present the Generalized Homogeneous Fredholm Equation (GHFE) [1]:

yemrω=ηeω0Knmωrr'yendr'E3

Eq. (3) has a special index ethat mean a specific resonance [1, 4, 5, 8, 9]. Among the three Eqs. (1), (2), and (3) there are a common ingredient, for each equation we have used different names: λ,νand η[1, 2, 3] but any of them can be incorporated to the kernel or used as an independent function or even an eigenvalue. In order to connect the homogeneous and inhomogeneous equation we must define some functions as we will see in the next sections.

## 3. Connection between the eigenvalues ηeand the functionλ

We know that because of the Hilbert-Schmidt theory [2, 3] and more recently by our previous results [1], the solutions of Eq. (3) that is all the yemrω, form a set of orthogonal functions and then a set of eigenvalues ηeω. Thus we can relate the functions appearing in Eqs. (1) and (3) as follows:

By means of the spectral representation of Green function, [2, 3] we have:

Gmnωrs=eCeyemryensληeE4

And also

umrω=umrωe=10yemrωyenr'ωλωηeωumr'ωdr'E5

The orthogonality relation is

0yemrωAmnyinrωdr=0ifieE6

## 4. Conditions imposed over the homogeneous Fredholm equations

In accordance with the theory of homogeneous Fredholm integral equations [1, 2, 13], the first Fredholm minor is a two point function, like a Green function, which must comply with an integral equation:

Mmrr0ω=ηωΔηω+ηω0KnmωrsMnsr0ωdsE7

Two other conditions must be satisfied:

The first is that Fredholm determinant is zero

Δηω=0E8

The second that the Fredholm eigenvalue equals to one:

ηω=1E9

But thanks to our second order approximation Eq. (2) we can show that other interesting conditions are satisfied, for example if we define some particular functions (and operators):

ΨrωMmrr0ωΔηωumrωE10

And also

ΨrωΔηωηωum(rω)+Δηωηωνω0Knmωrr'unr'ωdr'E11

We can see that the first Fredholm minor must satisfy through Ψ the inhomogeneous equation

ΨrωΨrω+ηω0KnmωrtΨtωdtE12

In order to write Eq. (2) in terms of the solutions of Eq. (1), we can define the operator:

Θmrων2ωεω0Knmωrr'0Klnωr'r''Pmr''ωdr''dr'E13

In Eq. (13) the function Pmrωis an arbitrary negative exponential regulator.

Near a resonance the two small parameters νωand εωmakes Θmrωlesser than a second order term, so can be neglected. Far of a resonance this later function sketches the behavior of the simultaneous existence of the resonant and non-resonant solutions because in terms of Θmrωthe conventional waves satisfy the inhomogeneous equation:

umrω=umrω+Θmrω+νω0Knmωrr'unr'ωdr'E14

## 5. Defining a new class of integral operators

As we said in Section 1, hidden in the structure of Eq. (2) there are some integral operators which allow the simultaneous existence of solutions with extremely different boundary conditions. So, let us define the Zap operators by the rules:

ZumrωZrωumrωumrω=+Δηηumorω+η0Knmωrr'unr'ωdr'E15

That is, the Zap operator is associated to the integral Fredholm equation satisfied by the affected solution (umrωor yemrω), from which takes the source term and the free kernel.

The same operator (15) acting over a homogeneous equation looks like

ZyemrωZrω0yemrω=+Δηη0+η0Knmωrr'yenr'ωdr'E16

That is

ZyemrωZrω0yemrω=λyemrωE17

As we can see the effect of the Zap operator is to kill or eliminate the inhomogeneous term when applied to a resonant state. But this seems very artificial because we are giving indeed two parts for the complete rule. However we can build projection operators that can make the work we need.

Now, we define Zap projection operators in the next section.

## 6. The zap projection operators and their properties

On this section, we define the so named Zap projection operators (ZPO) which enable us to project a complex broadcasting system over a reduced resonant simplest one. The Zap operators acts over Fourier transforms [14, 15] related to integral operators.

hen, based on (15) and (17), we define de following operator:

ZPrωumrω=limη1ZrωumrωE18

In order to get a display of the properties of this operator we propose a specific set of discrete antennas in the next example:

Suppose that we have ppunctual sources that can be represented in the inhomogeneous term of the Fredholm equation like:

umrω=i=1pαinδrriKnmωrriE19

Then, by applying the projection operator to Eq. (19) we have (remember that when η=1thus Δ=0):

ZPrωumrωumrω=limη1Δηηi=1pαinδrriKnm(ωrri)
+η0Knmωrr'unr'ωdr'E20

Now, because η=1implies Δ=0, and because also ν=λ=1

ZPrωumrωumrω=yemrωE21

In the last step we have used the fact that the solution of the remaining homogeneous equation is denoted by yemrω.

Eq. (21) says that if we take a blend of regular and resonant solutions we have:

ZPrωumrωumrω+yemrω=2yemrωE22

So taken into account from Eqs. (18) until (22), we see that we have projected the original problem into a resonant one.

In analogy with ZPwe can define a projector over their complement:

Let us define the complementary Zap projection operator as

ZQrωumrωumrωlimη1ZCrωumrωlimη1Δηη+ν0Knmωrr'unr'ωdr'+ηumrω=umrωE23

Even we apply ZQto a resonant state:

ZQrωumrωyemrωlimη1ZCrωumrω=limη1Δηη+ν0Knmωrr'yenr'ωdr'+ηumrω=umrωE24

This is because the name of the solution of the remaining inhomogeneous equation is precisely umrω.

## 7. An academic example for conventional traveling waves

In order to convince us of the utility of the ZPand ZQoperators we remember that in all of our developments the kernel always is Knmthat only contains the free Green function Gntωrs. But then, there is no difference between the kernels of the integral equations when are referred to conventional traveling waves or to evanescent or resonant waves. This last statement allows describing in an algebraic mode the application of the Zap projection operators. In this manner we can fix our kernel in accordance with a previous example that we have presented in some place as the matrix (27).

For the case of only two source points and omitting the three components of the field lifting only one, this matrix can be for example:

But first remember that

VAr'Gωrr'ur'ωdV'KωurωE25

In Eq. (25)Aris the interaction that in the general case may contain a non-local potential, but not in our example.

KωurωK11K12K21K22u1rωu2rωKωg1ωg2ωurE26

In Eq. (26)uris a scalar function.

And then, the kernel may be

Kω=sinωωpdωωpdicosωωpdωωpdicosωωpdωωpdsinωωpdωωpdE27

So Eq. (19) takes the form:

urω=i=12αiδrriKωeiE28

Where

e1=10ande2=01E29

That is

urω=α1δrr1Kωe1+α2δrr2Kωe2E30

The conventional waves satisfy the scalar form of Eq. (1)

urω=urω+λω0Kωrr'ur'ωdr'E31

Or in accordance with Eq. (25)

urω=urω+λωKωurωE32

Where the form of urωis unknown but possibly be sketched as

urω=sinωωpdωωpdsinωωp+βdωωp+βdurE33

Now we can apply the projection operator ZPrωurωto Eq. (32) and obtain

ZPrωurωurω=limη1Δηηi=12αiδrriKωei
+ηKωurωE34

Then, by putting η=1and Δ=0finally

ZPrωurωurω=yerωE35

So it is irrelevant the part of the problem concerning the two sources, it is only a problem about resonances. Our problem is now to find the resonant frequencies by taking Kωand impose the conditions η=1and Δ=0.

But, what is the real advantage of the ZPand ZQoperators?, the answer is that the Zap operator formalism may be viewed as a test for distinguish between an expression that cannot be transformed or yes, in whatever sense between the homogeneous and inhomogeneous equations under the rules established above; if not, we can ensure that some kind of irregular things are present. In case of the positive transformation, we have the confidence that both kinds of solutions can coexists, and then we can separate the solutions for convenience as if it was a problem of two steps: homogeneous and inhomogeneous.

Now the last condition over the Fredholm determinant is

Δsinωωpdωωpdηicosωωpdωωpdicosωωpdωωpdsinωωpdωωpdη=0E36

The parameters d, η(the Fredholm eigenvalue) and ωp(the plasma frequency) can take in principle, arbitrary values but for a specific media can be take numeric values. Now, we remember that we must also impose η=1.

Then, Eq. (36) has two resonances:

ω1=π4d+ωpE37

And

ω1=3π4d+ωpE38

## 8. Forerunners of the zap projection operators

In Section 6 we defined a new class of integral operators we named Zap projection operators that literally cleans from a broadcasting problem the inhomogeneous part and leaves a projected homogeneous version. These operators act directly over an inhomogeneous Fredholm equation and are related to the Fredholm operators. But recently, we have defined another set of operators we called generalized projection operators (GPO) which projects a complete broadcasting signal (maybe described by a GIFE) not only into several independent mutually orthogonal signals but also can reverse the time direction as we wish. These GPO may be considered as the precursors of the Zap operators and we will see why. We remember their form:

Ωe,T+SatT=eiωetPSaetE39

And

Ωe,TSat=eiωeTtPSaeTtE40

In Eqs. (39) and (40)PSaeTtare simple projection operators [11].

Denoting the Fourier transform like

FftFωE41

Then, the Fourier transform of the GPO is

FPSaeteiωet=n=Cn,e2ωep4πωe2ωωeωeeiωωeπnFSaωωeE42

Where p4πωe2ωωeωeis a rectangular function.

And for the convolution we have

FPSaeteiωetPSbeteiωut=FSaωωeFSbωωuE43

Then we see that the set of Fourier transforms of the GPOs behaves like a set of orthogonal basis functions for the frequency domain, that is, the resonant functions yemrωas we can verify in Figure 1. So the GPO can be considered as the forerunners of the Zap projection operators.

## 9. Conclusions

We can conclude that the Zap projection operators (ZPO) can be used as an alternative approach to the generalized projection operators (GPO) that is like an alternative for clean the evanescent signals [10] from disturbances generated by the sources and at the same time to clean the source signals from resonant solutions. We can also use the two classes of projectors in a consecutively manner. The former vision suppose that the evanescent waves [10] can be considered as part of the conventional traveling waves like an everything and that we must take away the effect of the resonances with the application of the ZQoperator. In any case we have shown the power of the Fourier transform applied to mathematical analysis in broadcasting problems and to physically characterize and solve them.

chapter PDF

## More

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## How to cite and reference

### Cite this chapter Copy to clipboard

Juan Manuel Velazquez Arcos, Ricardo Teodoro Paez Hernandez, Alejandro Perez Ricardez, Jaime Granados Samaniego and Alicia Cid Reborido (November 5th 2020). On the Zap Integral Operators over Fourier Transforms [Online First], IntechOpen, DOI: 10.5772/intechopen.94573. Available from: