Three most classical, hyperbolic systems of two equations in one-dimension are the system of isentropic gas dynamics in Eulerian coordinates

where is the density of gas, the velocity and the pressure; the nonlinear hyperbolic system of elasticity

where denotes the strain, is the stress and the velocity, which describes the balance of mass and linear momentum, and is equivalent to the nonlinear wave equation

and the system of compressible fluid flow

To obtain the global existence of weak solutions for nonstrictly hyperbolic systems (two eigenvalues are real, but coincide at some points or lines), the compensated compactness theory (cf. [1] [2] or the books [3] -[5] ) is still a powerful and unique method until now.

For the polytropic gas where and is an arbitrary positive constant, the Cauchy problem (1) with bounded initial data was completely resolved by many authors (cf. [6] -[11] ). When has the same principal singularity as the -law in the neighborhood of vacuum, a compact framework was first provided in [12] [13] and later, the necessary compactness of weak entropy-entropy flux pairs for general pressure function was completed in [14] .

Under the strictly hyperbolic condition and some linearly degenerate conditions or as, the global existence of weak bounded solutions, or solutions, was obtained by Diperna [15] and Lin [16] , Shearer [17] respectively.

Without the strictly hyperbolic restriction, a preliminary existence result of the nonlinear wave Equation (3) was proved in [18] for the special case under the assumption or.

Using the Glimm’s scheme method (cf. [19] ), Diperna [20] first studied the system (4) in a strictly hyperbolic region. Roughly speaking, for the polytropic case, Diperna’s results cover the case.

Since the solutions for the case of always touch the vacuum, its existence was obtained in [21] by using the compensated compactness method coupled with some basic ideas of the kinetic formulations (cf. [10] [11] ). The existence of the Cauchy problem (7) for more general function was given in [22] under some conditions to ensure the compactness for all smooth entropy-entropy flux pairs.

If all smooth entropy-entropy flux pairs satisfy the compactness, an ideal compactness framework to prove the global existence was provided by Diperna in [15] . For the above three systems (1)-(2) and (4), we can prove the compactness only for half of the entropies (weak or strong entropy).

In [14] (see also [23] for inhomogeneous system), the author constructed a sequence of regular hyperbolic systems

to approximate system (1), where in (5) denotes a regular perturbation constant and the perturbation pressure

The most interesting point of this kind approximation is that both systems (5) and (1) have the same entropies (or the same entropy equation). In [14] , the compactness of weak entropy-entropy flux pairs was also proved for general pressure function.

Let the entropy-entropy flux pairs of systems (1) and (5) be and

respectively. Then by using Murat-Tartar theorem, we have

for any fixed, where the weak-star limit is denoted by as goes to zero.

Paying attention to the approximation function (6), we know that

are the entropy-entropy flux pairs of system

or system

respectively.

If we could prove from the arbitrary of in (7) that

and

where denotes the weak-star limit as tend to zero, then we would have more function Equations (12) to reduce the strong convergence of as tend to zero.

Between systems (2) and (4), we have the following approximation

which has also the same entropy equation like system (2). If we could prove (11) and (12) from (7), then similarly we could prove the equivalence of systems (2) and (4). Moreover, we have much more information from system (13) to prove the existence of solutions for system (2) or (4).

Systems (13) and (2) have many common basic behaviors, such as the nonstrict hyperbolicity, the same entropy equation, same Riemann invariants and so on.

By simple calculations, two eigenvalues of system (13) are

with corresponding right eigenvectors

and Riemann invariants

Moreover

and

Any entropy-entropy flux pair of system (13) satisfies the additional system

Eliminating the from (19), we have

Therefore systems (13) and (2) have the same entropies. From these calculations, we know that system (13) is strictly hyperbolic in the domain or, while it is nonstrictly hyperbolic on the domain since when.

However, from (17) and (18), for each fixed, both characteristic fields of system (13) are genuinely nonlinear in the domain if or in the domain if

. In the first case, we have an a-priori estimate for the solutions of system (13)

because the region

is an invariant region, where (is given in Theorem 1), and are positive constants depending on the initial date, but being independent of. In the second case, we have the estimate

because the region

is an invariant region.

In this paper, for fixed, we first establish the existence of entropy solutions for the Cauchy problem (13) with bounded measurable initial data

In a further coming paper, we will study the relation between the functions equations (11) and (12), and the convergence of approximated solutions of system (13) as goes to zero.

Theorem 1 Suppose the initial data be bounded measurable. Let (I):

where is a positive constant, or (II):. Then the Cauchy problem (13)

with the bounded measurable initial data (23) has a global bounded entropy solution.

Note 1. The idea to use the flux perturbation coupled with the vanishing viscosity was well applied by the author in [24] to control the super-line, source terms and to obtain the estimate for the nonhomogeneous system of isentropic gas dynamics.

Note 2. It is well known that system (2) is equivalent to system (1), but (1) is different from system (4) although the latter can be derived by substituting the first equation in (1) into the second. However, (4) can be considered as the approximation of (2). In fact, let in (13). Then (13) is rewritten to the form

for some nonlinear function.

Note 3. For any fixed, the invariant region above is bounded, so the vacuum is avoided. However, the limit of, as goes to zero, is the original invariant region of system (2) because could be infinity from the estimates in (21).

In the next section, we will use the compensated compactness method coupled with the construction of Lax entropies [25] to prove Theorem 1.

In this section, we prove Theorem 1.

Consider the Cauchy problem for the related parabolic system

with the initial data (23).

We multiply (25) by and, respectively, to obtain

and

Then the assumptions on yield

and

if; or

and

if

If we consider (28) and (29) (or (30) and (31)) as inequalities about the variables and, then we can get the estimates by applying the maximum principle to (28) and (29) (or

by applying the maximum principle to (30) and (31)). Then, using the first equation in (25), we get or depending on the conditions on. Therefore, the region

or

is respectively an invariant region. Thus we obtain the estimates given in (21) or (22) respectively.

It is easy to check that system (13) has a strictly convex entropy when or

We multiply (4.1) by to obtain the boundedness of

in. Then it follows that

is bounded in. Since for some bounded constants

when or, we get the boundedness of

for any fixed.

Now we multiply (4.1) by, where is any smooth entropy of system (13), to obtain

where is the entropy-flux corresponding to. Then using the estimate given in (35), we know that the first term in the right-hand side of (36) is compact in, and the second is bounded in

. Thus the term in the left-hand side of (36) is compact in.

Then for smooth entropy-entropy flux pairs of system (13), the following measure equations or the communicate relations are satisfied

where is the family of positive probability measures with respect to the viscosity solutions of the Cauchy problem (25) and (23).

To finish the proof of Theorem 1, it is enough to prove that Young measures given in (37) are Dirac measures.

For applying for the framework given by DiPerna in [5] to prove that Young measures are Dirac ones, we construct four families of entropy-entropy flux pairs of Lax’s type in the following special form:

where are the Riemann invariants of system (13) given by (16). Notice that all the unknown functions are only of a single variable. This special simple construction yields an ordinary differential equation of second order with a singular coefficient before the term of the second order derivative. Then the following necessary estimates for functions are obtained by the use of the singular perturbation theory of ordinary differential equations:

uniformly for or, where and is a positive constant independent of.

In fact, substituting entropies into (20), we obtain that

Let

and

Then

The existence of and its uniform bound on or with respect to can be obtained by the following lemma (cf. [26] ) (also see Lemma 10.2.1 in [15] ):

Lemma 2 Let be the solution of the equation

and functions be continuous on the regions

for some positive functions and. In addition,

for some positive constants and.

If is a solution of the following ordinary differential equation of second order:

with and being arbitrary, then for sufficiently small and , exists for all and satisfies

where

Furthermore, we can use Lemma 2 again to obtain the bound of with respect to if we differentiate Equation (46) with respect to.

By the second equation in (19), an entropy flux corresponding to is provided by

where

if or, and both are bounded uniformly on or.

In a similar way, we can obtain estimates on another three pairs of entropy-entropy flux of Lax type. Hence, Theorem 1 is proved when we use these entropy-entropy flux pairs in (38)-(41) together with the theory of compensated compactness coupled with DiPerna’s framework [15] .

In this paper we have looked at the general system of one-dimensional nonlinear elasticity in Lagrangian coordinates (2).

We construct a hyperbolic approximations to this which are parameterized by. They all have the same entropies as the original system. Under suitable assumptions we are able to establish uniform compactness estimates, and then obtain the existence of entropy solutions for the Cauchy problem.

This work was partially supported by the Natural Science Foundation of Zhejiang Province of China (Grant No. LY12A01030 and Grant No. LZ13A010002) and the National Natural Science Foundation of China (Grant No. 11271105).