Αυθεντικό κείμενο - Αγγλικά - itareted series

Given infinitely many real numbers indexed by two integers j and k, imagine these numbers arranged in an infinite rectangular matrix.
Suppose we first sum the elements in each row of (1) separately, obtainingan infinite sequence of series of the form..... andthen sum the terms of this squence in turn, obtaining the expression...called an itareted series.