# Trefftz method

One of the variational methods for solving boundary value problems. Suppose one has to solve the boundary value problem

$$ \tag{* } \Delta u = 0,\ \ u \mid _ {S} = \phi , $$

where $ S $ is the boundary of a domain $ \Omega \subset \mathbf R ^ {m} $. The solution of the problem (*) minimizes the functional

$$ J ( u) = \int\limits _ \Omega ( \mathop{\rm grad} u ( x)) ^ {2} dx $$

over all functions satisfying the boundary condition $ u \mid _ {S} = \phi $. Trefftz' method consists in the following. Suppose one is given a sequence of harmonic functions $ w _ {1} , w _ {2} \dots $ in $ \Omega $ that are square summable in $ \Omega $ together with their first derivatives. An approximate solution is sought in the form

$$ u _ {n} = \ \sum _ {j = 1 } ^ { n } c _ {j} w _ {j} , $$

the coefficients $ c _ {j} $ being determined from the condition that $ J ( u _ {n} - u) $ is minimal, where $ u $ is the exact solution of (*). This leads to the following system of equations for $ c _ {1} \dots c _ {n} $:

$$ \sum _ {j = 1 } ^ { n } c _ {j} \int\limits _ { S } w _ {j} \frac{\partial w _ {i} }{\partial \nu } \ dS = \ \int\limits _ { S } \phi \frac{\partial w _ {i} }{\partial \nu } dS,\ \ i = 1 \dots n, $$

where $ \nu $ is the outward normal to $ S $.

Trefftz' method can be generalized to various boundary value problems (see [2]–[4]).

The method was proposed by E. Trefftz (see [1]).

#### References

[1] | E. Trefftz, "Ein Gegenstück zum Ritzschen Verfahren" , Verhandl. 2er Internat. Kongress. Techn. Mechanik Zürich, 1926, 12–17 Sept. , O. Füssli (1927) pp. 131–137 |

[2] | S.G. [S.G. Mikhlin] Michlin, "Variationsmethoden der mathematischen Physik" , Akademie Verlag (1962) (Translated from Russian) |

[3] | V.I. Krylov, V.V. Bobkov, P.I. Monastyrnyi, "Computing methods of higher mathematics" , 2 , Minsk (1975) (In Russian) |

[4] | M.Sh. Birman, "Variational methods for solving boundary value problems analogous to Trefftz' method" Vestnik Leningrad. Gos. Univ. Ser. mat. Mekh. i Astr. , 11 : 13 (1956) pp. 69–89 (In Russian) |

#### Comments

#### References

[a1] | K. Rektorys (ed.) , Applicable mathematics , Iliffe (1969) pp. 1056–1058 |

**How to Cite This Entry:**

Trefftz method.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Trefftz_method&oldid=49030