Appendix

A natural question occurs when removing a common assumption like regular sampling: Has it been discreetly incorporated into some mathematical result which may be used unknowingly? This is why I will present a derivation of the 444#444 and 429#429 approximations of the wave equation, starting from the basic laws of mechanics. The derivation will not ``open'' the Laplacian in (), keeping it as an invariant to the very end. The derivation is not original: it is based on () and ().