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Let \(\Delta\) be the forward difference operator, \(\nabla\) be the backward difference operator and \(\delta\) be the central difference operator. Then :

  • A.\(\nabla \Delta = \Delta - \nabla\)
  • B.\(\nabla \Delta \neq \delta^{2}\)
  • C.\(\Delta - \nabla \neq \delta^{2}\)
  • D.\(\nabla \Delta \neq \Delta - \nabla\)

Answer: A

The correct relation is \(\nabla \Delta = \Delta - \nabla\).

Write the operators with the shift E: \(\Delta = E - 1\), \(\nabla = 1 - E^{-1}\), \(\delta = E^{1/2} - E^{-1/2}\). Then

\[ \begin{aligned} \nabla\Delta &= (1 - E^{-1})(E - 1) = E - 2 + E^{-1} \\ \Delta - \nabla &= (E - 1) - (1 - E^{-1}) = E - 2 + E^{-1} \\ \delta^{2} &= E - 2 + E^{-1} \end{aligned} \]

So \(\nabla\Delta = \Delta - \nabla = \delta^{2}\). That makes option A true and options B, C and D, each of which asserts one of these equalities fails, false.

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