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Bargmann-Type Inequality for Half-Linear Differential Operators
Journal of Inequalities and Applications volume 2009, Article number: 104043 (2009)
Abstract
We consider the perturbed half-linear Euler differential equation ,
,
, with the subcritical coefficient
. We establish a Bargmann-type necessary condition for the existence of a nontrivial solution of this equation with at least
zero points in
.
1. Introduction
The classical Bargmann inequality [1] originates from the nonrelativistic quantum mechanics and gives an upper bound for the number of bound states produced by a radially symmetric potential in the two-body system. In the subsequent papers, various proofs and reformulations of this inequality have been presented, we refer to [2, Chapter XIII], and to [3–5] for some details.
In the language of singular differential operators, Bargmann's inequality concerns the one-dimensional Schrödinger operator
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ1_HTML.gif)
It states that if the Friedrichs realization of has at least
negative eigenvalues below the essential spectrum (what is equivalent to the existence of a nontrivial solution of the equation
having at least
zeros in
), then
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ2_HTML.gif)
where .
This inequality can be seen as follows. The Euler differential equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ3_HTML.gif)
with the subcritical coefficient is disconjugate in
, that is, any nontrivial solution of (1.3) has at most one zero in this interval. Hence, if the equation
, with
given by (1.1), has a solution with at least
positive zeros, the perturbation function
must be "sufficiently positive" in view of the Sturmian comparison theorem. Inequality (1.2) specifies exactly what "sufficient positiveness" means.
In this paper, we treat a similar problem in the scope of the theory of half-linear differential equations:
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ4_HTML.gif)
In physical sciences, there are known phenomena which can be described by differential equations with the so-called -Laplacian
, see, for example, [6]. If the potential in such an equation is radially symmetric, this equation can be reduced to a half-linear equation of the form (1.4).
There are many results of the linear oscillation theory, which concern the Sturm-Liouville differential equation:
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ5_HTML.gif)
which has been extended to (1.4). In particular, the linear Sturmian theory holds almost verbatim for (1.4), see, for example, [7, 8]. We will recall elements of the half-linear oscillation theory in the next section. Our main result concerns the perturbed half-linear Euler differential equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ6_HTML.gif)
where is a continuous function, and shows that if
is the so-called subcritical coefficient, that is,
, and there exists a solution of (1.6) with at least
zeros in
, then the integral
satisfies an inequality which reduces to (1.2) in the linear case
.
2. Preliminaries
In this short section, we present some elements of the half-linear oscillation theory which we need in the proof of our main result. As we have mentioned in the previous section, the linear and half-linear oscillation theories are in many aspects very similar, so (1.4) can be classified as oscillatory or nonoscillatory as in the linear case.
If is a solution of (1.4) such that
is some interval
, then
is a solution of the Riccati-type differential equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ7_HTML.gif)
If (1.4) is nonoscillatory, that is, (2.1) possesses a solution which exists on some interval , among all such solutions of (2.1), there exists the minimal one
, minimal in the sense that any other solution
of (2.1) which exists on some interval
satisfies
in this interval, see [9, 10] for details.
In our treatment, the so-called half-linear Euler differential equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ8_HTML.gif)
appears. If we look for a solution of this equation in the form , then
is a root of the algebraic equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ9_HTML.gif)
By a simple calculation (see, e.g., [8, Section 1.3]), one finds that (2.3) has a real root if and only if is less than or equal to the so-called critical constant
, and hence (2.2) is nonoscillatory if and only if
. In this case, the associated Riccati equation is of the form
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ10_HTML.gif)
and its minimal solution is , where
is the smaller of (the two real) roots of (2.3). If
, then
is a solution of the equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ11_HTML.gif)
and is the minimal solution of this equation. A detailed study of half-linear Euler equation and of its perturbations can be found in [11].
3. Bargmann's Type Inequality
In this section, we present our main results, the half-linear version of Bargmann's inequality. We are motivated by the work in [4] where a short proof of this inequality based on the Riccati technique is presented. Here we show that this method, properly modified, can also be applied to (1.6).
Theorem 3.1.
Suppose that (1.6) with has a nontrivial solution with at least
zeros in
Then
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ12_HTML.gif)
where is the absolute value of the difference of the real roots of
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ13_HTML.gif)
and is the conjugate number to
Moreover, the constant
is strict in the sense that for every
, there exists a continuous function
such that (1.6) possesses a solution with
zeros in
and
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ14_HTML.gif)
Proof.
Let be a solution of (1.6) with
zeros in
denote these zeros by
, and let
Then by a direct computation we see that
is a solution of the Riccati-type differential equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ15_HTML.gif)
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ16_HTML.gif)
Let be the roots of (3.2). Such pair of roots exists and it is unique since the function
is convex,
,
, and
According to (3.5), there exist
such that
,
, and
for
, which means that
for
Then, we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ17_HTML.gif)
Now we prove that the constant is exact. Let
be arbitrary and
be sequences of positive real numbers constructed in the following way. Let
be arbitrary and consider the differential equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ18_HTML.gif)
Denote by its nontrivial solution satisfying
,
(such solution exists and it is unique, see, e.g., [8, Section 1.1]) and let
Since
, see [8, page 39], there exists
such that
.
Now, let
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ19_HTML.gif)
and define for the function
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ20_HTML.gif)
Consider the solution of the equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ21_HTML.gif)
given by the initial conditions . Then for
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ22_HTML.gif)
Hence,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ23_HTML.gif)
Now consider again (3.7) and the associated Riccati-type differential equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ24_HTML.gif)
(which is related to (3.7) by the substitution ). This equation has a constant solution
and this solution is the minimal one (see the end of Section 2). This means that any solution of (3.13) which starts with the initial condition
blows down to
at a finite time
, which is a zero point of the associated solution
of (3.7). Now, let
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ25_HTML.gif)
In summary, we have constructed a solution of the equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ26_HTML.gif)
for which and
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ27_HTML.gif)
The construction of and
is now analogical. As a result we obtain the function
defined as
for
and
, and
for
, for which
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ28_HTML.gif)
and the equation
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ29_HTML.gif)
has a solution with zeros at
Finally, we change the discontinuous function to a continuous one
such that
Such a modification is an easy technical construction which can be described explicitly, but for us is only important its existence. According to the Sturmian comparison theorem, the equation
possesses a nontrivial solution with at least
zeros and
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ30_HTML.gif)
which we needed to prove.
Remark 3.2.
If , then
and the roots of (3.2) are
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F104043/MediaObjects/13660_2009_Article_1886_Equ31_HTML.gif)
Hence, and (3.1) reduces to (1.2).
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Acknowledgment
The authors thank the referees for their valuable remarks and suggestions which contributed substantially to the present version of the paper. The first author is supported by the Grant OTKA CK80228 and the second author is supported by the Research Project MSM0021622409 of the Ministry of Education of the Czech Republic and the Grant 201/08/0469 of the Grant Agency of the Czech Republic.
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Bognár, G., Došlý, O. Bargmann-Type Inequality for Half-Linear Differential Operators. J Inequal Appl 2009, 104043 (2009). https://doi.org/10.1155/2009/104043
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DOI: https://doi.org/10.1155/2009/104043