#### Abstract

A first order of accuracy difference scheme for the
approximate solution of abstract nonlocal boundary value problem , , , , for differential equations in a Hilbert space with a self-adjoint positive definite operator *A* is considered. The well-posedness of this difference scheme in HΓΆlder spaces without a weight is established. Moreover, as applications, coercivity estimates in HΓΆlder norms
for the solutions of nonlocal boundary value problems for elliptic-parabolic equations are obtained.

#### 1. **Introduction**

Nonlocal boundary value problems for partial differential equations have been applied by various researchers in order to model numerous processes in different fields of applied sciences when they are unable to determine the boundary values of the unknown function (see, e.g., [1β15] and the references therein).

Well-posedness of difference schemes of elliptic-parabolic equations with nonlocal boundary conditions in HΓΆlder spaces with a weight was studied in [16β19].

In paper [20], the well-posedness of abstract nonlocal boundary value problem in HΓΆlder spaces without a weight was established. The coercivity inequalities for solutions of the boundary value problem for elliptic-parabolic equations were obtained.

In the present paper, the first order of accuracy difference scheme for the approximate solution of problem (1.1) is considered. The well-posedness of difference scheme (1.2) in HΓΆlder spaces without a weight is established. As an application, coercivity inequalities for solutions of difference scheme for elliptic-parabolic equations are obtained.

Throughout the paper, denotes a Hilbert space and is a self-adjoint positive definite operator with for some . Then, it is wellknown that is a self-adjoint positive definite operator and . Furthermore, and which are defined on the whole space , are bounded operators, where is the identity operator.

#### 2. **Well-Posedness of (1.2)**

First of all, let us start with some auxiliary lemmas that are used throughout the paper.

Lemma 2.1. *The following estimates are satisfied [19, 21, 22]:
**
for some , which is independent of is a positive small number.*

Let be the linear space of mesh functions defined on with values in the Hilbert space . Next, , , , and denote Banach spaces on with norms:

With the help of the self-adjoint positive definite operator in a Hilbert space , the Banach space consists of those for which the norm (see [22, 23]): is finite. By the definition of , for all .

Lemma 2.2. *For , the norms of the spaces and are equivalent (see [24]).*

Theorem 2.3. *Suppose , , and . Boundary value problem (1.2) is wellposed in HΓΆlder space and the following coercivity inequality holds:
**
where is independent of not only , , andββ but also of and .*

*Proof. *First of all, let us get the formulae for solution of problem (1.2). By [21, 25],
is the solution of boundary value difference problem:
is the solution of inverse Cauchy problem:

Combining the conditions and formulas (2.6), (2.8), we get formulas

Operator equation
follows from formulas (2.10), (2.11), and the condition . As the operator
has an inverse
it follows that
for the solution of operator equation (2.12). Hence, we have formulas (2.10), (2.11), and (2.15) for the solution of difference problem (1.2).

Using formulae (2.10) and (2.15), we can get

Finally, we will get coercivity estimate (2.5). It is based on estimates
for the solution of boundary value difference problem (2.7),
for the solution of inverse Cauchy difference problem (2.9), and
for the solution of problem (1.2). Estimates (2.18) and (2.19) were established in [21, 25], respectively.

Estimates (2.20) are derived from the formulas (2.16) and (2.17) for the solution of problem (1.2), estimates (2.1) and following estimates
which were established in [26]. This finalizes the proof of Theorem 2.3.

#### 3. **An Application**

In this section, an application of the abstract Theorem 2.3 is considered. First, let be the unit open cube in the -dimensional Euclidean space with boundary . In , the mixed boundary value problem for multidimensional mixed equation: is considered. Here, , and are given smooth functions and .

The discretization of problem (3.1) is carried out in two steps. In the first step, the grid sets

are defined. To the differential operator generated by problem (3.1), the difference operator is assigned by formula: acting in the space of grid functions , satisfying the conditions for all . With the help of , we arrive at the nonlocal boundary-value problem for an infinite system of ordinary differential equations.

In the second step, problem (3.4) is replaced by difference scheme (1.2) (see [21]): To formulate the result, we introduce the Hilbert spaces , and of the grid functions defined on , equipped with the norms:

Theorem 3.1. *Let and be sufficiently small numbers. Then, the solutions of difference scheme (3.5) satisfy the following coercivity stability estimate:
**
where is not dependent on , and .*

The proof of Theorem 3.1 is based on Theorem 2.3, the symmetry properties of the difference operator defined by formula (3.3), and along with the following theorem on the coercivity inequality for the solution of elliptic difference equation in .

Theorem 3.2. *For the solution of elliptic difference problem:
**
the following coercivity inequality holds [27]:
**
Here, depends neither on nor .*

#### Acknowledgments

The author would like to thank Professor Allaberen Ashyralyev (Fatih University, Turkey) for his inspirational contributions and to the anonymous referees whose careful reading of the paper and valuable comments helped to improve it.