^{1} Law of reflection also is given as one of the local minima of T, but in that case *c*(*x*, *y*, *z*) is constant and much of the ‘fun’ is lost.

^{2} It is important to appreciate that the boundary conditions inflict almost no damage to the arbitrariness of *δy*. Boundary conditions are only *2n* in number, whereas *δy*(*x*) has infinite coordinates in (*x*_{1},*x*_{2}).

^{3} For a three-variable problem, one needs to use divergence theorem at this stage.

^{4} or, a string, in line with our analogy in a similar context, in Chap. 30

^{5} Note that *yi* is involved in *two* of the terms in the summation.

^{6} For example, the residual of the Poisson’s equation *u*_{xx}+*u*_{yy} = *h*(*x*, *y*) operated over

^{7} For that matter, not every differential equation possesses an underlying variational ...

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