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ABSTRACT
In this paper we study a two-phase size-structured population model with distributed delay in the birth process. This model distinguishes individuals by ‘active’ or ‘resting’ status. The individuals in the two life-stages have different growth rates. Only individuals in the ‘active’ stage give birth to the individuals in the ‘active’ stage or the ‘resting’ stage. The size of all the newborns is 0. By using the method of semigroups, we obtain that the model is globally well-posed and its solution possesses the property of asynchronous exponential growth. Moreover, we give a comparison between this two-phase model with the corresponding one-phase model and show that the asymptotic behaviours of the sum of the densities of individuals in the ‘active’ stage and the ‘resting’ stage and the solution of the corresponding one-phase model are different.
1. Introduction
In this paper, we study a two-phase size-structured population model with distributed delay in the birth process. The individuals in this model are distinguished by two distinct life-stages: the ‘active’ stage and the ‘resting’ stage. Only individuals in the ‘active’ stage give birth to the individuals in the ‘active’ stage or the ‘resting’ stage. The individuals in the two life-stages have different growth rates. The size of all the newborns is 0. The delay in this model is given by the time lag between conception and birth or laying and hatching of the parasite eggs (see [Citation2,Citation17]). Moreover, unlike the non-distributed delay case, the time lag considered here can change from 0 to the maximal value, i.e. it is distributed in an interval. We denote by and
the densities of individuals in the ‘active’ stage and the ‘resting’ stage, respectively, of size
at time
, where
represents the finite maximal size any individual may reach in its lifetime. Then the model reads as follows:
(1)
(1) Here the vital rates
and
are the size-dependent growth rates and mortality rates of individuals in the ‘active’ stage and the ‘resting’ stage, respectively, the functions
and
are the size-dependent rates of transition between the ‘active’ stage and the ‘resting’ stage, the function
represents the rate that an individual of size x in the ‘active’ stage reproduces after a time lag
starting from conception, where τ is a constant denoting the maximal delay, and ν is a constant,
. In addition,
and
are given functions defined in
and
, respectively. Later on we shall denote
(2)
(2)
The one-phase model with delay in the birth process and the growth rate was studied in [Citation15]. The global well-posedness and the property of so called asynchronous exponential growth (see [Citation1,Citation7,Citation8–12,Citation15] for the definition) of its solution were obtained by using the method of semigroups. The author of [Citation11] gave a completely different proof for the property of asynchronous exponential growth which can generalize to more complicated one-phase size-structured models with the growth rate
by using the method of characteristics. Recently, hopf bifurcation was obtained in the similar one-phase model with two delays in [Citation13]. The model considered here distinguishes individuals by two different stages and the two stages may have different growth rates. In this paper, we shall prove that under suitable assumptions on
,
,
,
,
,
, β and
, the model (Equation1
(1)
(1) ) is globally well-posed and its solution possesses the property of asynchronous exponential growth. Moreover, we shall give a comparison between this two-phase model with the corresponding one-phase model by the method which is inspired by [Citation14]. More precisely, we shall consider the special case where
and shows that the asymptotic behaviours of the sum of the densities of individuals in the ‘active’ stage and the ‘resting’ stage and the solution of the corresponding one-phase model are different. This implies that the research on the model which distinguishes individuals by the ‘active’ stage and the ‘resting’ stage are meaningful. In fact, the asynchronous exponential growth of solutions of size-structured population models with two stages have been investigated by many authors including [Citation9], [Citation12], and [Citation3–5]. In contrast with those model, the model considered here is a non-local boundary condition problem which contains distributed delay. We use the methods of Hille–Yosida operators and isomorphic operators. The comparison between the two-phase model with the corresponding one-phase model has been also considered in [Citation3,Citation5]. The conjugate problem here is different because of the non-local boundary condition and the distributed delay.
Throughout this paper, ,
,
,
,
,
, and
are supposed to satisfy the following conditions:
,
,
, and
are non-negative and continuous functions defined on
.
,
, and
,
for all
.
and
.
and
.
In order to prove the property of asynchronous exponential growth, we make the additional assumptions:
for all
and
.
If
,
for all
. If
,
for all
.
We introduce the subspace of
and the subspace
of
as follows:
where
is defined in (Equation2
(2)
(2) ).
Our first main result establishes the global well-posedness of the model (Equation1(1)
(1) ) and reads as follows:
Theorem 1.1
For any , the model (Equation1
(1)
(1) ) has a unique solution
, and for any T>0, the mapping
from
to
is continuous.
The proof of this result will be given in Section 2.
From the proof of Theorem 1.1, we shall have that for any , the solution of the model (Equation1
(1)
(1) ) can be expressed as
, where
is a strongly continuous semigroup on the space
. Our second main result studies the asymptotic behaviour of the solution of the model (Equation1
(1)
(1) ) and reads as follows:
Theorem 1.2
There exist a rank projection Π on and constants
,
,
such that
where
denotes the operator norm on
.
The proof of this result will be given in Section 3. The parameter is called intrinsic rate of natural increase or Malthusian parameter (see [Citation16]). This result shows that the solution of the model (Equation1
(1)
(1) ) exhibits asynchronous exponential growth.
Next we consider the special case of the model (Equation1(1)
(1) ), where
and give a comparison between this two-phase model with the corresponding one-phase model. Note that the above result means that there exists a positive vector function
, such that for any
, the solution
of the model (Equation1
(1)
(1) ) has the following asymptotic expression:
(3)
(3) where
is a constant uniquely determined by the initial data
. We denote
(4)
(4)
(5)
(5) We can see that
is the asymptotic proportion of the individuals in the ‘active’ stage in the population. Then we have the following problem which describes the evolution of the sum of the densities of individuals in the ‘active’ stage and the ‘resting’ stage in the asymptotic sense:
(6)
(6) where
. By using the same method, we have that the solution
of the model (Equation6
(6)
(6) ) has the following asymptotic expression:
where
is a constant uniquely determined by the initial data
.
Let
(7)
(7) and
(8)
(8) We can see that
is the sum of the densities of individuals in the ‘active’ stage and the ‘resting’ stage. From (Equation3
(3)
(3) ), we have the following asymptotic expression:
Then we want to compare
and the solution of the model (Equation6
(6)
(6) ). One might expect that
. But this is actually not the case. In fact, we have the following result:
Theorem 1.3
Let the notation be as above. We have the following relation:
where c is a constant which is generally non-vanishing.
The proof of this result will be given in Section 3. This result shows that the asymptotic behaviours of the sum of the densities of individuals in the ‘active’ stage and the ‘resting’ stage and the solution of the one-phase model are different and the research on the model with two stages is meaningful.
The layout of the rest of the paper is as follows. In Section 2, we reduce the model (Equation1(1)
(1) ) into an abstract Cauchy problem and establish the well-posedness of it by means of strongly continuous semigroups. In Section 3, we prove that the solution of the model (Equation1
(1)
(1) ) exhibits asynchronous exponential growth. In Section 4, we compare this two-phase model with the corresponding one-phase model and give the proof of Theorem 1.3.
2. Reduction and well-posedness
In this section we reduce the model (Equation1(1)
(1) ) into an abstract Cauchy problem and establish the well-posedness of it. Since this model is a non-local boundary condition problem which contains distributed delay, we use the methods of Hille-Yosida operators and isomorphic operators(see [Citation15]).
First, we introduce the following operators on the Banach spaces and
:
where
We note that
,
and
. Using these notations, we rewrite the model (Equation1
(1)
(1) ) into the following abstract initial value problem for a retarded differential equation on X:
(9)
(9) where
and
are defined as
and
, respectively, and
is defined as
Next, we introduce the following operators on E:
We note that
and
. We now let
, and introduce operator
on
as follows:
We note that
. Using these notations, we see that problem (Equation9
(9)
(9) ) can be equivalently rewrite into the following abstract initial value problem of an ordinary differential equation on
:
(10)
(10) where
and
.
Remark 2.1
As usual, if the functions and
satisfy
and
, respectively, and problem ([Equation9
(9)
(9) ]), we say that functions
is a classical solution of problem ([Equation9
(9)
(9) ]). It is evident that a necessary condition for problem ([Equation9
(9)
(9) ]) to have a classical solution is that
and
.
Remark 2.2
As usual, we say that a function is a classical solution of problem ([Equation10
(10)
(10) ]) if
and it satisfies ([Equation10
(10)
(10) ]) in usual sense.
To be rigorous, we write down the following preliminary result:
Lemma 2.1
Let the necessary condition mentioned in Remark 2.1 is satisfied. If
,
is a classical solution of problem ([Equation9
(9)
(9) ]), then
is a classical solution of problem ([Equation10
(10)
(10) ]). Conversely, if
is a classical solution of problem ([Equation10
(10)
(10) ]), then
has the form
for all
, and by extending the first component of its second component
to
such that
for
, we have that
is a classical solution of problem ([Equation9
(9)
(9) ]).
Proof.
See Lemma 2.2 of [Citation15] and Lemma 2.1 of [Citation6].
We consider the Banach space and the operator
with domain
, where
is the projection onto the first coordinate.
Lemma 2.2
The operator is a Hille-Yosida operator on the Banach space
.
Proof.
The operator can be written as the sum of two operators on
as
, where
with
and
. Since
is a bounded operator on
, by Lemma 3.1 of [Citation15], it suffices to prove that
is a Hille–Yosida operator on the Banach space
.
The restriction of G to the kernel of Q generates the nilpotent left shift semigroup
on E, given by
(11)
(11) Similarly, the restriction
of A to the kernel of P generates the strongly continuous semigroup
E, given by
(12)
(12) where
where
and
The resolvent of
is
where
for
,
and
for
,
. The resolvent set
of
contains
. For
and
,
From the proof of Proposition 3.2 of [Citation15], we have that
. Since
generates a contraction semigroup on X, by Theorem II.3.5 of [Citation10], we have that
. A direct calculation shows that
and
, where
for
.
Therefore, we have that and
is a Hille–Yosida operator on the Banach space
. This completes the proof.
By Proposition 5.9 of [Citation15], we have the following result.
Lemma 2.3
The part of
in the closure of its domain
generates a strongly continuous semigroup.
Lemma 2.4
The operator is isomorphic to the part
of the operator
in the closure of its domain
.
Proof.
We have
Therefore, the operator is isomorphic to the part
of the operator
in the closure of its domain
. We also refer the reader to see Theorem 3.3 of [Citation15] for the similar proof.
By Lemma 2.3, Lemma 2.4 and Theorem II.6.7 of [Citation10], we have the following result.
Theorem 2.5
The operator generates a strongly continuous semigroup
on
. For any given initial data
, problem (Equation10
(10)
(10) ) has a unique solution
, given by
(13)
(13)
By Lemma 2.1 and Theorem 2.5, we see that Theorem 1.1 follows.
3. Asynchronous exponential growth
In this section we study the asymptotic behaviour of the solution ofproblem ([Equation1(1)
(1) ]). We shall prove that the semigroup
has the property of asynchronous exponential growth on
by using Theorems 9.10 and 9.11 of [Citation8]. To this end, we will prove that the semigroup
is an irreducible positive strongly continuous semigroup (see Definition II.1.7 and Theorem VI.1.2 of [Citation10] for the definitions) satisfying the inequality
, where
and
are the essential growth bound and the growth bound of the semigroup
generated by
(see Definition IV.2.1 and Definition IV.2.9 of [Citation10] for the definitions). In contrast with the semigroups in [Citation3–5,Citation9,Citation12], the generator
contains the non-local boundary condition and the distributed delay, we use the isomorphic operators in the proof of the property of asynchronous exponential growth.
We first deduce an useful expression of . For
, let
. Then
satisfies the equation
(14)
(14) By writing
and
, we see that Equation (Equation14
(14)
(14) ) can be rewritten as follows:
Then
(15)
(15)
(16)
(16)
(17)
(17) where
(18)
(18) and
(19)
(19)
For each , we define two operators
and
on
as follows:
(20)
(20)
(21)
(21) where
Since
there exists
such that
for
. This implies that the inverse
exists and is a bounded operator for
. From (Equation15
(15)
(15) )–(Equation21
(21)
(21) ) we see that the resolvent of
is given by
(22)
(22)
Lemma 3.1
The semigroup generated by
is positive.
Proof.
From Theorem VI.1.8 of [Citation10], the desired assertion follows if we prove that the resolvent of its generator
is positive for all sufficiently large λ. From (Equation20
(20)
(20) )–(Equation22
(22)
(22) ), we have that
for all sufficiently large λ. This completes the proof.
Lemma 3.2
The semigroup generated by
is irreducible.
Proof.
By Lemma VI.1.9 of [Citation10], if we prove that for some
and all
and ∀
such that
and
, then the desired assertion then follows. Let
and
be the projections onto the first and the second coordinates, respectively. We use the expression (Equation22
(22)
(22) ) to prove that
for almost all
,
and
for almost all
. The process of this proof is similar to Lemma 3.4 in [Citation3] and Lemma 3.4 in [Citation5]. We omit it here.
Lemma 3.3
.
Proof.
The operator can be written as the sum of two operators on
as
, where
with
and
. Since
can be written as the sum of
and a bounded operator
, where
with
,
is a Hille–Yosida operator on the Banach space
(see Lemma 3.1 of [Citation15]). By Proposition 4.4 of [Citation15], we have that
of
in
generates a strongly continuous semigroup on
which is isomorphic to the semigroup
generated by
with domain
By Proposition 4.4 of [Citation15], we have that
where
,
are defined in (Equation11
(11)
(11) ) and (Equation12
(12)
(12) ), and
are linear operators defined as
where
is the projection onto the first coordinate. By Lemma 3.6 of [Citation12], we have that
for
, where
. From the definition
(see Definition IV.2.9 of [Citation10]),
. Since
can be restricted to
and is a bounded and compact operator on
, by Theorem 4.5 of [Citation15] and Lemma 2.3, we have that
. This completes the proof.
Lemma 3.4
, where
is the spectral bound of
, i.e.
.
Proof.
Since the semigroup generated by
is positive, we have that
(see Theorem VI.1.15 of [Citation10]). We split the operator
into the sum of two operators:
, where
Then we define the operator
as follows:
By Corollary VI.1.11 of [Citation10], we have that
. Then the desired assertion follows if we prove that
. To this end, we consider the eigenvalue problem
(23)
(23) By writing
, we see that Equation (Equation23
(23)
(23) ) can be rewritten as follows:
Then
(24)
(24) where
and
are defined in (Equation18
(18)
(18) ) and (Equation19
(19)
(19) ). Multiplying Equation (Equation24
(24)
(24) ) by
and integrating from
to 0 and 0 to
with respect to σ and x, respectively, we have that the eigenvalue equation (Equation23
(23)
(23) ) admits a non-trivial eigenvector if and only if
, where
The function
, restricted to R, is continuous, strictly decreasing, with
and
. Therefore, the equation
has a unique real zero
, which implies that
. This completes the proof.
By Lemmas 3.1–3.4, we conclude that is a strictly dominant eigenvalue of generator
(see Corollary V.3.2 of [Citation10]) and the fist-order pole of
with a one-dimensional residue
and there exist the constants
and
such that
(25)
(25) where
denotes the operator norm in
(see Theorem 4.1 of [Citation15], Theorems 9.10 and 9.11 of [Citation8], and Theorem C-IV.2.1 of [Citation1]). This completes the proof of Theorem 1.2.
4. Relation with the one-phase model
In this section we compare the two-phase model with the corresponding one-phase model and give the proof of Theorem 1.3 by the method which is inspired by [Citation14]. This kind of comparison has been also considered in [Citation3,Citation5]. The conjugate problem here is different because of the non-local boundary condition and the distributed delay.
From (Equation25(25)
(25) ), we have the following asymptotic expression:
(26)
(26) as
, where
is a constant uniquely determined by the initial data
,
and
are the dominant eigenvalue and the corresponding eigenvector of the eigenvalue problem
(27)
(27) We can see that the corresponding eigenvector
is strictly positive, i.e.
for all
,
and
for all
.
Let be the solution of the model (Equation6
(6)
(6) ). Similarly, we have the following asymptotic expression:
(28)
(28) where
is a constant uniquely determined by the initial data
,
and
are the dominant eigenvalue and the corresponding eigenvector of the eigenvalue problem
(29)
(29) where
,
and
are defined in (Equation4
(4)
(4) ) and (Equation5
(5)
(5) ). The corresponding eigenvector
is also strictly positive, i.e.
for all
,
for all
. From (Equation4
(4)
(4) ), (Equation5
(5)
(5) ), (Equation27
(27)
(27) ) and (Equation29
(29)
(29) ), we have
and
.
Next we want to compare and
, where
is defined in (Equation7
(7)
(7) ) and (Equation8
(8)
(8) ). The asymptotic expression (Equation26
(26)
(26) ) implies that
(30)
(30) From (Equation28
(28)
(28) ) and (Equation30
(30)
(30) ), we have that
where
and
. Then we prove that
.
Let be the eigenvector of the conjugate problem of Equation (Equation27
(27)
(27) ), i.e.
(31)
(31) We normalize
such that
(32)
(32) Due to a similar reason as that for
,
is strictly positive, i.e.
for all
,
and
for all
.
From (Equation1(1)
(1) ) and (Equation31
(31)
(31) ), we easily obtain that
Hence
for all
. Letting
and using (Equation26
(26)
(26) ), we have that
From (Equation32
(32)
(32) ), we have that
(33)
(33)
Let be the eigenvector of the conjugate problem of Equation (Equation29
(29)
(29) ), i.e.
(34)
(34) We normalize
such that
(35)
(35)
is also strictly positive, i.e.
for all
,
for all
. By a similar argument we have that
so that generally speaking we have
or
. This proves Theorem 1.3.
Acknowledgments
The authors are glad to acknowledge their gratefulness to the anonymous reviewers for valuable comments and suggestions on modifying this manuscript.
Disclosure statement
No potential conflict of interest was reported by the authors.
Additional information
Funding
References
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