The measured equation of invariance (MEI) is a concept in field computation which leads to the generation of local linear equations in lieu of absorption conditions to terminate finite difference or finite element equations. The advantage of the method is that the equations are not limited to replacing absorbing conditions, but can be used where there are both outgoing and incoming waves. In fact, they can be used in closed or semiclosed environments such as cavities or waveguides. Yet unlike the integral equation or boundary element method, which results in global matrices, MEI gives local equations and sparse matrices. The savings in storage and computing time by MEI over conventional methods are very substantial.

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