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The solutions of the linear equations are represented in a real vector spaceM. A radius of M, which represents proportional solutions, is called a "mode".[clarification needed]
A norm may be defined. For instance, in electromagnetism, it is usually the energy of the solution assuming that there is no other field in S. From the norm are defined the orthogonality and the scalar product of solutions. The orthogonality of solutions extends to the corresponding modes.