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## Homework Statement

Consider the wave operator

[tex] \Box^2 = \frac{\partial^2}{\partial t^2} - \frac{\partial^2}{\partial x^2}[/tex]

Write Green's forumula for an arbitrary domain [itex] \Omega [/itex] with boundary [itex] \Gamma [/itex]

## Homework Equations

define a vector q, where

[tex] q \cdot e_t = n \cdot e_t [/tex]

[tex] q \cdot e_x = -n \cdot e_x [/tex]

where e_x, e_t are unit vectors in x, t dimensions, n is the normal.

Show Green's formula takes the form

[tex] \int_\Omega (v\Box^2u - u\Box^2 v)dx dt = \int_\Gamma \left( v\frac{\partial u}{\partial q} -u\frac{\partial u}{\partial q} \right)dl[/tex]

where dl is an element of arclength along the bounding curve [itex] \Gamma [/itex]

(u and v are arbitrary functions in R1)

## The Attempt at a Solution

I'm starting with Green's formula for an arbitrary linear operator:

[tex] \int_\Omega (vLu - L^*v) = \int_\Gamma div(J(u,v)) [/tex]

Where L is an ODE operator, and J is the conjunct of u and v. For the operator above, this takes the form of

[tex] \int_\Omega (vLu - L*v) = \int_\Gamma div \left[e_t \left( v\frac{\partial u}{\partial t} -u\frac{\partial u}{\partial t}\right) + u grad_x v - v grad_x u \right][/tex]

Where grad_x is the gradient along x. Now, the div operator should give us the same a the dot product of the normal to the line space:

[tex] = \int_\Gamma n\cdot\left[e_t \left( v\frac{\partial u}{\partial t} -u\frac{\partial u}{\partial t}\right) + u grad_x v - v grad_x u\right] [/tex]

I believe that [itex] n \cdot grad_x = \frac{\partial}{\partial n}[/itex]. So, substituting the form of q above, I have

[tex] = \int_\Gamma \left( q\cdot e_t \left( v\frac{\partial u}{\partial t} -u\frac{\partial u}{\partial t}\right) + \left(v\frac{\partial u}{\partial q} - u\frac{\partial u}{\partial q}\right)\right) [/tex]

I'm almost there, I'm just not sure how [itex] \left( q\cdot e_t \left( v\frac{\partial u}{\partial t} -u\frac{\partial u}{\partial t}\right) [/itex] disappears.