#### Abstract

A class of reaction-diffusion systems unifying several Aedes aegypti population dynamics models is considered. Equivalence transformations are found. Extensions of the principal Lie algebra are derived for some particular cases.

#### 1. Introduction

In this paper we focus our attention on the following class of nonlinear advection-reaction-diffusion systems: These systems can describe the evolution of the densities and of two interacting populations where the balance equation for takes into account not only the reaction-diffusion effects but also some advection effects while the balance equation for density takes into account only the so-called reaction terms. The advection effects are due to the presence in the function of the gradient and appear when the individuals of population feel external stimuli as, for instance, wind effects or water currents. Class (1) can be considered a generalization of the equations with the typical properties of the already known Aedes aegypti mathematical models [1ā3]. We recall them shortly in the following.

The Aedes aegypti mosquitos are the main vector of dengue, a viral disease that causes the so-called dengue hemorrhagic fever characterized by coagulation problems often leading the infected individual to death. Since the subtropical zone climate and environmental conditions are favorable to the development of Aedes aegypti, dengue is a serious public health problem in many countries around the world. However, due to the global warming, the interest in such considered mosquitoes is not restricted to those places affected by the disease, but it is also of interest for those countries whose weather, in the next decades, can become similar to the current environmental found in the subtropical zone. Therefore, the interest in modeling such a vector is not only a theoretical deal but also a way for finding methods and alternatives to overcome and control the problems arising from the dispersal dynamics of the mosquitos and, consequently, the propagation of the disease.

The following system where , belongs to class (1). It was introduced in [2] as a generalization of a model studied in [1].

We recall that in (2), as well as in [1], and are, respectively, nondimensional densities of winged and aquatic populations of mosquitoes and , , , and are nondimensional, in general, positive parameters, . Specifically is the ratio between two constants and , which are, respectively, the carrying capacity related to the amount of findable nutrients and the carrying capacity effect dependent on the occupation of the available breeder and denotes the specific rate of maturation of the aquatic form into winged female mosquitoes, while and are, respectively, the mortality of winged and the mortality of aquatic populations. Finally denotes a constant of velocity for flux due to wind currents that, in general, generate an advection motion of large masses of the winged population and consequently can facilitate a quick advance of the infestation. For further details, see [4] and references therein.

Another system belonging to (1) is which was also introduced in [2] starting from (2) and, due to the weak interaction between the aquatic and winged populations, by modifying the source terms.

The first equation of (3) gives the time rate of change of the mosquitoes density as a sum of the growth terms , of the per capita death rate and the diffusive-advective flux due to the movement of mosquito population.

The second equation gives the corresponding time rate of change of the density of aquatic population as a sum of the growth term of aquatic population, due to the new egg depositions of female mosquitoes, with per capita death rate of aquatic population. The term represents the loss due to the change of the aquatic into winged form; see [4].

The family of system (1) contains arbitrary functions or numerical parameters, which specifies the individual characteristics of phenomena belonging to large subclasses. In this sense, the knowledge of equivalence transformations can provide us with certain relations between the solutions of different phenomena of the same class and allows us to get symmetries in a quite direct way.

Following [5], an equivalence transformation is a nondegenerated change of independent and dependent variables , , , and into , , , and : which maps a system of class (1) in another one of the same class, that is, in an equation preserving differential structure but, in general, with

Of course in the case an equivalence transformation becomes a symmetry.

In this paper we look for certain equivalence transformations for the class of systems (1) in order to find symmetries for special systems belonging to (1) and to get information about constitutive parameters , , and appearing there. Moreover we wish to stress that, as it is known, an equivalence transformation maps solutions of an equation in solutions of the transformed equation [6]. Then in order to find solutions for a certain equation one can look for the equivalence transformations that bring the equation in simpler other ones whose solutions are well studied; see, for example, [7, 8] and references inside.

The plan of the paper is as follows. In the next section we provide some elements about equivalence transformations. In Section 3 we apply these concepts in order to obtain a set of weak equivalence generators. In Section 4, after having introduced a projection theorem, we show how to apply it to find symmetries of (1). In Section 5, after having introduced a special structure of the advection-reaction function that generalizes that one used in (3), we find extensions with respect to the principal Lie algebra. Conclusions and final remarks are given in Section 6.

#### 2. Elements on Equivalence Transformations

In the past differential equation literature it is possible to find several examples of equivalence transformations. The direct search for the most general equivalence transformations through the finite form of the transformation is connected with considerable computational difficulties and quite often leads to partial solutions of the problem (e.g., [9, 10]).

A systematic treatment to look for continuous equivalence transformations by using the Lie infinitesimal criterion was suggested by Ovsiannikov [11].

In general, the equivalence transformations for class (1) can be considered as transformations acting on point of the basic augmented space

The previous elements allow us to consider, in the following, the one-parameter equivalence transformations as a group of transformations, acting on the basic augmented spaceāā, of the type which is locally a -diffeomorphism, depending analytically on the parameter in a neighborhood of , and reduces to the identity transformation for .

Following [6, 11ā14] (see also, e.g., [5, 15ā17] ) we consider the infinitesimal generator of the equivalence transformations (8) of the systems (1) that reads as follows: where the infinitesimal components , , , and are sought depending on , , , and , while the infinitesimal components are sought, at least in principle, depending on , , , , , , , , , , , and . In order to obtain the determining system which allows us to get the infinitesimal coordinates , , and ( and ), we apply the Lie-Ovsiannikov infinitesimal criterion by requiring the invariance, with respect to suitable prolongations and of (9), of the following equations: together with the invariance of the auxiliary conditions [13, 14, 18, 19]: where and are functions while , , and are considered as functions depending, a priori, on . All of these functions are assumed to be analytical. The constraints, given by (11), characterize the functional dependence of , , and .

In this paper, instead, in view of further applications and following [20], we modify the previous classical procedure by looking for equivalence transformations whose generators are got by solving the determining system obtained from the following invariance conditions: As the functional dependences of the parameters , , and are known a priori we do not require the invariance of the auxiliary conditions (11). In this way we work in a basic augmented spaceāā. Therefore the components must be sought, at least in principle, depending on , , , , , , , and .

The infinitesimal operators, obtained by following this shortening procedure, can generate transformations that map equations of our class into new equations of the same class where the transformed arbitrary functions may have new additional functional dependencies. Such transformations are called weak equivalence transformations [13, 14].

With respect to the application in biomathematical models, equivalence and weak equivalence transformations were applied not only to study of tumor models [21, 22] but also to the population dynamics in [20, 23, 24].

#### 3. Calculation of Weak Equivalence Transformations

In order to avoid long formulas and write and in a compact way, we put For this reason system (1) is rewritten as while the equivalence generator assumes the following form: where , , and . Here the summation over the repeated indices is presupposed.

After putting the prolongations and assume the following form: where

The invariant conditions read both under the constraints (15), which are after (18)

The coefficients , , , and are given, respectively, by

Taking into account (24) and (27), we can write with the constraints (25) and (26).

Then, substituting (25) and (26) into (28), we get

From this condition we obtain the following determining equations: Then it follows that and, consequently, .

Following the same procedure we can write the invariance condition (23) as

Collecting the terms with and with and equating their respective coefficients to zero it is obtained that,

Thus, from the coefficients of and Considering the coefficients of we get The remaining terms of (32) give the following form for the infinitesimal component of : Therefore, once having taken into account all restrictions obtained we are finally able to write the following infinitesimal components for the weak equivalence generators: while, from (36), we get where , , , and are arbitrary real functions of their arguments. Then, going back to the original variables, the most general operator of these continuous weak equivalence transformations reads

#### 4. Symmetries for the System (1)

In the next sections in order to carry out symmetries for the system (1) we do not use the classical Lie approach. Instead of the mentioned method we apply the projection theorem, introduced in [25] and eventually reconsidered in [13, 14, 18, 19]. In agreement with these references, we can affirm the following.

Theorem 1. Let be an infinitesimal equivalence generator for the system (1); then the operator which corresponds to the projection ofāā on the space , is an infinitesimal symmetry generator of the system (1) if and only if the constitutive equations, specifying the forms of , , and , are invariant with respect to .

For the system under consideration, in general, the constitutive equations whose invariance must be requested are The request of invariance brings us to the following equations: under the restrictions (42).

Then substituting and taking into account the constraints we can write (44) as We recall here that the principal Lie algebra [5, 12] is the Lie algebra of the principal Lie group, that is, the group of the all Lie point symmetries that leave the system (1) invariant for any form of the functions , , and . In other words we can remark that the principal Lie algebra is the subalgebra of the equivalence algebra such that any operator of this subalgebra leaves the equations , , and invariant for any form of the functions , , and . Then we can say [5] the following.

Corollary 2. An equivalence operator for the system (1) belongs to the principal Lie algebraāā if and only if , , , and .

Taking Corollary 2 into account from the previous equations (46)-(48) it is a simple matter to ascertain that the [5, 12] is spanned by the following translation generators:

#### 5. Some Extensions of

In order to show some extensions of the principal algebra, which could be of interest in biomathematics, we assume that the advection-reaction function is of the form where the parameters , , , , , , and are constitutive parameters of the considered phenomena.

This form of is a generalization of appearing in (3), where Consequently in (51) we must consider and as limit cases and . Moreover, in this section, we assume that the value will not be considered because in this case the advective effects disappear. We also assume that . This last restriction implies that the balance equation of the density depends on the density . Finally for the sake of simplicity we omit the limit case and assume that the diffusion is only nonlinear; that is, .

In the following we continue the discussion of invariance conditions written in the previous section.

From (46), by deriving with respect to , we get Then with and arbitrary constants, so (46) becomes while, after having taken (51) into account, (47) reads From (57) we get immediately

In the following we analyze separately the case and the case .

##### 5.1.

From coefficient we get ; that is,

Therefore, from the remaining terms we have

As we assumed , from the coefficient of in (60) we conclude that and Then, still from (60) we have the following constraints to consider: From (62) two cases are obtained.

(i) Case . Then, from (62) we conclude that and it follows that From (64) and (61) we obtain with and arbitrary constants.

The analysis of (63) leads to the following two subcases.

āConsiderā, with Taking into account the previous results and going back to (56) and (48) we get that the system (3) with of the form (51) admits the 3-dimensional Lie algebra spanned by the translations in space and time and by the following additional generator: provided that and are solutions of the following differential equations:

āConsiderā. In this case the only symmetries admitted are translations of the independent variables and the form of and is arbitrary, so there is not an extension of the principal Lie algebra.

(ii) Case . In this case from (62) it follows that After having substituted (69) into (61) we obtain with , , and arbitrary constants and once assumed . Finally we analyze the contribution of (63) from where the following two subcases arise.

āāConsider , with Then taking into account the previous results it reads Going to put the previous result in the condition (46) and by separating the variable we get with and arbitrary constants and provided that the diffusion coefficient is solution of From condition (48) we do not get further restrictions on the infinitesimal components of the symmetry generator but only the following constraint on the reaction function : Taking into account the arbitrariness of in this case we have got a 3-dimensional Lie algebra. The additional generator is In particular, from (75), setting , we obtain , while from (75) we obtain the power function .

Remark 3. By putting , the corresponding system of class (1) admits still a 3-dimensional symmetry Lie algebra generated by translations in time and in space and by . These results are in agreement with the ones obtained in [2].

By considering we do not get extension of for and arbitrary. We get, instead, the following extension: provided that . and .

āConsiderā. In this case the only symmetries admitted are the translations in space and time and the form of and is arbitrary.

##### 5.2.

We analyze this case by beginning with the discussion of (56) from which two cases arise.

āā Is Arbitrary.āāIt follows, of course, that and so with arbitrary constant.

Moreover (46) and (48) become But taking into account that (79) becomes and gives us the following conditions: from where, taking into account the work hypotheses at the beginning of this section, we get with arbitrary function of and .

In this case the only admitted symmetries are translations in time and space.

Consider . Then by requiring we get from where we derive

(a) arbitrary function and and then and , so we omit this case,

(b) In this case which implies where Equation (47) after (54), (58), and (81) becomes From terms in we get the following condition: which gives us and then . Therefore, from (87) which implies that

From (92) arise two cases.

(i) Consider and . In this case after having derived from (91) the following additional conditions it is a simple matter to ascertain that there does not exist extension of .

(ii) Consider with

Then (91) assumes the following form:

For , from (97) we obtain the following:

From the previous conditions we consider the following subclasses

(A) Consider and . As a consequence we get and then there is no extension of .

(B) For and we get, for , and then in this subcase we got an extension by one of given by provided that and are solutions of the following equations: For , instead, we conclude that Therefore, the extension is given by provided that and are solutions of the following differential equations:

#### 6. Conclusions

In this paper we have considered a class of reaction-diffusion systems with an additional advection term. The studied class includes, as particular cases, all partial differential equation models concerned with the Aedes aegypti mosquito that have been proposed until now. We have investigated such a system from the point of view of equivalence transformations in the spirit of the Lie-Ovsiannikov algorithm based on the Lie infinitesimal criterion. In agreement with some modifications of the Lie-Ovsiannikov algorithm, introduced in [20], we obtained a group of weak equivalence transformations. We have applied these transformations in order to obtain symmetry generators by using a projection theorem. In particular after having obtained the principal Lie algebra, we investigated a specific, but quite general, form for the advection-reaction term and we derived some extensions of the principal Lie algebra. The specializations of the results for the systems studied in [2] are in agreement with those ones obtained there.

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgments

The authors would like to thank FAPESP for financial support (Grants 2011/20072-0 and 2011/19089-6). Mariano Torrisi would also like to thank CMCC-UFABC for its warm hospitality and GNFM (Gruppo Nazionale per Fisica-Matematica) for its support. He also thanks the support from the University of Catania through PRA.