Lagrange's identity (boundary value problem)
In the study of ordinary differential equations and their associated boundary value problems, Lagrange's identity, named after Joseph Louis Lagrange, gives the boundary terms arising from integration by parts of a self-adjoint linear differential operator. Lagrange's identity is fundamental in Sturm–Liouville theory. In more than one independent variable, Lagrange's identity is generalized by Green's second identity.
The operator L and its adjoint operator L* are given by:
where S is the surface bounding the volume Ω and n is the unit outward normal to the surface S.
Ordinary differential equationsEdit
Any second order ordinary differential equation of the form:
can be put in the form:
This general form motivates introduction of the Sturm–Liouville operator L, defined as an operation upon a function f such that:
It can be shown that for any u and v for which the various derivatives exist, Lagrange's identity for ordinary differential equations holds:
where , , and are functions of . and having continuous second derivatives on the interval .
Proof of form for ordinary differential equationsEdit
The leading multiplied u and v can be moved inside the differentiation, because the extra differentiated terms in u and v are the same in the two subtracted terms and simply cancel each other. Thus,
which is Lagrange's identity. Integrating from zero to one:
as was to be shown.
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