# Partial differential equations, property C for

Property stands for "completeness" of the set of products of solutions to homogeneous linear partial differential equations. It was introduced in [a1] and used in [a2], [a3], [a4], [a5], [a6], [a7], [a8], [a9], [a10], [a11], [a12], [a13] as a powerful tool for proving uniqueness results for many multi-dimensional inverse problems, in particular, inverse scattering problems (cf. also Inverse scattering, multi-dimensional case).

Let be a bounded domain in , , let , where is a multi-index, , derivatives being understood in the distributional sense, the , , are certain functions, is the null-space of the formal differential operator , and the equation is understood in the distributional sense.

Consider the subsets and for which the products are defined, , .

The pair has property if and only if the set is total (complete) in , ( is fixed), that is, if and

then .

By property one often means property or with any fixed .

Is property generic for a pair of formal partial differential operators and ?

For the operators with constant coefficients, a necessary and sufficient condition is given in [a10] for a pair to have property . For such operators it turns out that property is generic and holds or fails to hold simultaneously for all : Assume . Denote , . Note that , .

Therefore if and only if .

Define the algebraic varieties (cf. also Algebraic variety)

One says that is transversal to , and writes , if and only if there exist a point and a point such that the tangent space to (in ) at the point and the tangent space to at the point are transversal (cf. Transversality).

The following result is proved in [a1]: The pair of formal partial differential operators with constant coefficients has property if and only if .

Thus, property fails to hold for a pair of formal differential operators with constant coefficients if and only if the variety is a union of parallel hyperplanes in .

Therefore, property for partial differential operators with constant coefficients is generic.

If and the pair has property , then one says that has property .

## Examples.

Let , . Then . It is easy to check that there are points and at which the tangent hyperplanes to are not parallel. Thus has property . This means that the set of products of harmonic functions in a bounded domain is complete in , (cf. also Harmonic function). Similarly one checks that the operators

have property .

Numerous applications of property to inverse problems can be found in [a1].

Property holds for a pair of Schrödinger operators with potentials , , where is the set of functions with compact support{} (cf. also Schrödinger equation).

If , , , , is the unit sphere in , are the scattering solutions corresponding to the Schrödinger operators , , , then the set of products , is fixed, is complete in , where is an arbitrary fixed bounded domain [a1]. The set , where is fixed, is total in the set , where is the Sobolev space [a1].

#### References

[a1] | A.G. Ramm, "Multidimensional inverse scattering problems" , Longman/Wiley (1992) |

[a2] | A.G. Ramm, "Scattering by obstacles" , Reidel (1986) |

[a3] | A.G. Ramm, "Completeness of the products of solutions to PDE and uniqueness theorems in inverse scattering" Inverse Probl. , 3 (1987) pp. L77–L82 |

[a4] | A.G. Ramm, "Multidimensional inverse problems and completeness of the products of solutions to PDE" J. Math. Anal. Appl. , 134 : 1 (1988) pp. 211–253 (Also: 139 (1989), 302) |

[a5] | A.G. Ramm, "Recovery of the potential from fixed energy scattering data" Inverse Probl. , 4 (1988) pp. 877–886 (Also: 5 (1989), 255) |

[a6] | A.G. Ramm, "Multidimensional inverse problems: Uniqueness theorems" Appl. Math. Lett. , 1 : 4 (1988) pp. 377–380 |

[a7] | A.G. Ramm, "Multidimensional inverse scattering problems and completeness of the products of solutions to homogeneous PDE" Z. Angew. Math. Mech. , 69 : 4 (1989) pp. T13–T22 |

[a8] | A.G. Ramm, "Property C and uniqueness theorems for multidimensional inverse spectral problem" Appl. Math. Lett. , 3 (1990) pp. 57–60 |

[a9] | A.G. Ramm, "Completeness of the products of solutions of PDE and inverse problems" Inverse Probl. , 6 (1990) pp. 643–664 |

[a10] | A.G. Ramm, "Necessary and sufficient condition for a PDE to have property C" J. Math. Anal. Appl. , 156 (1991) pp. 505–509 |

[a11] | A.G. Ramm, "Property C and inverse problems" , ICM-90 Satellite Conf. Proc. Inverse Problems in Engineering Sci. , Springer (1991) pp. 139–144 |

[a12] | A.G. Ramm, "Stability estimates in inverse scattering" Acta Applic. Math. , 28 : 1 (1992) pp. 1–42 |

[a13] | A.G. Ramm, "Stability of solutions to inverse scattering problems with fixed-energy data" Rend. Sem. Mat. e Fisico (2001) pp. 135–211 |

**How to Cite This Entry:**

Partial differential equations, property C for. A.G. Ramm (originator),

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