... non-vanishing

This requirement is related to the requirement for the normal $\mathbf{A}^{T} \mathbf{A}$ operator, inroduced in the next section, to be a pseudo-differential operator (Wong, 1991). Situations where this condition is violated require a special consideration (Stolk, 2000; Nolan and Symes, 1996).

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...

If this is not the case, a different parameterization of the stacking function is appropriate (Fomel, 2001a)

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