which can also be written as
(To use this nice operator notation, one has to allow for the indices to start from n = 2 on, or apply a translation operator which first shifts the sequence indices by two, or to adopt the convention that xn = 0 for all n < 0.)
Obviously, Ax is ill-defined if Δ²x contains a zero element, or equivalently, if the sequence of first differences has a repeating term. From a theoretical point of view, assuming that this occurs only for a finite number of indices, one could easily agree to consider the sequence Ax restricted to indices n>n0 with a sufficiently large n0. From a practical point of view, one does in general rather consider only the first few terms of the sequence, which usually provide the needed precision. Moreover, when numerically computing the sequence, one has to take care to stop the computation when rounding errors become too important in the denominator, where the Δ² operation may cancel too many significant digits.
When converges linearly to , then
is not a linear operator, but a constant term drops out, viz: , if is a constant. This is clear from the expression of in terms of the finite difference operator .
Although the new process does not in general converge quadratically, it can be shown that for a fixed point process, that is, for an iterated function sequence for some function , converging to a fixed point, the convergence is quadratic. In this case, the technique is known as Steffensen's method.
Empirically, the A-operation eliminates the "most important error term". One can check this by considering a sequence of the form , where
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