The strong zero theorem for an elliptic boundary value problem in an angle

Sufficient algebraic conditions are given under which the solution of a homogeneous elliptic boundary value problem with constant coefficients in an angle, which has a zero of infinite order at the vertex, vanishes identically. If the angle equals $\pi$ or $2\pi$, the sufficient conditions are satisfied by all elliptic boundary value problems. The same is true in the case of an arbitrary angle if the principal part of the elliptic operator is a power of a second order operator.
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