2.2. Summation convention

By P<sup>β</sup> <sup>α</sup> f nð Þ, we mean the sum of all values of f nð Þ for which α ≤ n ≤ β. In the case β < α, then we take this to be zero.

Summations are over 0, 1, 2, …, when there is no indication to the contrary. If ð Þ¼ xk ð Þ x1; x2;… is a sequence of terms, then, by P <sup>k</sup> xk we mean <sup>P</sup><sup>∞</sup> <sup>k</sup>¼<sup>1</sup> xk and we shall sometimes write as <sup>P</sup>xk incase where no possible confusion arises.
