# Second boundary value problem

One of the boundary value problems (cf. Boundary value problem, partial differential equations) for partial differential equations. For example, let there be given a second-order elliptic equation

$$\tag{* } Lu = \sum _ {i, j = 1 } ^ { n } a _ {ij} ( x) \frac{\partial ^ {2} u ( x) }{\partial x _ {i} \partial x _ {j} } + \sum _ {i = 1 } ^ { n } b _ {i} ( x) \frac{\partial u ( x) }{\partial x _ {i} } +$$

$$+ c ( x) u ( x) = f ( x),$$

where $x = ( x _ {1} \dots x _ {n} )$, $n \geq 2$, in a bounded domain $\Omega$, with a normal at each point of the boundary $\Gamma$. The second boundary value problem for equation (*) in $\Omega$ is the following problem: Out of the set of all solutions of equation (*), isolate those solutions which have, at all boundary points, derivatives with respect to the interior normal $N$ and which satisfy the condition

$$\left . \frac{\partial u ( x, t) }{\partial N ( x) } \right | _ {x \in \Gamma } = \phi ( x),$$

where $\phi ( x)$ is a given function. The second boundary value problem is also known as the Neumann problem.

#### References

 [1] A.V. Bitsadze, "Boundary value problems for second-order elliptic equations" , North-Holland (1968) (Translated from Russian) [2] V.S. Vladimirov, "Equations of mathematical physics" , MIR (1984) (Translated from Russian) [3] C. Miranda, "Partial differential equations of elliptic type" , Springer (1970) (Translated from Italian) [4] I.G. Petrovskii, "Partial differential equations" , Saunders (1967) (Translated from Russian)