# Poly-harmonic function

*hyper-harmonic function, meta-harmonic function, of order *

A function of real variables defined in a region of a Euclidean space , , having continuous partial derivatives up to and including the order and satisfying the poly-harmonic equation everywhere in :

where is the Laplace operator. For one obtains harmonic functions (cf. Harmonic function), while for one obtains biharmonic functions (cf. Biharmonic function). Each poly-harmonic function is an analytic function of the coordinates . Some other properties of harmonic functions also carry over, with corresponding changes, to poly-harmonic functions.

For poly-harmonic functions of any order , representations using harmonic functions are generalized to get results known for biharmonic functions [1]–[5]. For example, for a poly-harmonic function of two variables there is the representation

where , , are harmonic functions in . For a function of two variables to be a poly-harmonic function, it is necessary and sufficient that it be the real (or imaginary) part of a poly-analytic function.

The basic boundary value problem for a poly-harmonic function of order is as follows: Find a poly-harmonic function in a region that is continuous along with its derivatives up to and including the order in the closed region and which satisfies the following conditions on the boundary :

(*) |

where is the derivative along the normal to and are given sufficiently smooth functions on the sufficiently smooth boundary . Many studies deal with solving problem (*) in the ball in [1], [6]. To solve the problem (*) in the case of an arbitrary region, one uses methods of integral equations, as well as variational methods [1], [6].

#### References

[1] | I.N. Vekua, "New methods for solving elliptic equations" , North-Holland (1967) (Translated from Russian) |

[2] | I.I. Privalov, B.M. Pchelin, "Sur la théorie générale des fonctions polyharmoniques" C.R. Acad. Sci. Paris , 204 (1937) pp. 328–330 Mat. Sb. , 2 : 4 (1937) pp. 745–758 |

[3] | M. Nicolesco, "Les fonctions poly-harmoniques" , Hermann (1936) |

[4] | M. Nicolesco, "Nouvelles recherches sur les fonctions polyharmoniques" Disq. Math. Phys. , 1 (1940) pp. 43–56 |

[5] | C. Tolotti, "Sulla struttura delle funzioni iperarmoniche in pui variabili independenti" Giorn. Math. Battaglini , 1 (1947) pp. 61–117 |

[6] | C. Miranda, "Partial differential equations of elliptic type" , Springer (1970) (Translated from Italian) |

#### Comments

See [a1] for an updated bibliography and for a slightly more general definition: is poly-harmonic on the domain if locally uniformly on .

#### References

[a1] | N. Aronszain, T.M. Creese, L.J. Lipkin, "Polyharmonic functions" , Clarendon Press (1983) |

[a2] | P.R. Garabedian, "Partial differential equations" , Chelsea, reprint (1986) |

**How to Cite This Entry:**

Poly-harmonic function.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Poly-harmonic_function&oldid=12716