“…As a result, another technique known as the …nite series technique, invented in 1988 [28], [29] and given up the same year due to the success of localized approximations techniques, was born again in 2019 in the case of Laguerre-Gauss beams freely propagating [30], or focused by a lens [31]. The paper by Votto et al summarized in the present preface [32] implements the …nite series technique for Laguerre-Gauss beams focused by a lens, complements another paper [33] devoted to the case of Laguerre-Gauss beams freely propagating, and constitutes some kind of conclusion in the series told in the present paragraph.…”