The problem of ordering operators has afflicted quantum mechanics since its foundation. Several orderings have been devised, but a systematic procedure to move from one ordering to another is still missing. The importance of establishing relations among different orderings is demonstrated by Wick's theorem (which relates time ordering to normal ordering), which played a crucial role in the development of quantum field theory. We prove the General Ordering Theorem (GOT), which establishes a relation among any pair of orderings, that act on operators satisfying generic (i.e. operatorial) commutation relations. We expose the working principles of the GOT by simple examples, and we demonstrate its potential by recovering two famous algebraic theorems as special instances: the Magnus expansion and the Baker-Campbell-Hausdorff formula. Remarkably, the GOT establishes a formal relation between these two theorems, and it provides compact expressions for them, unlike the notoriously complicated ones currently known.
Lieb-Mattis ordering theorem of electronic energy levels in the thermodynamic limit
University of Granada（格拉纳达大学）；Institute Carlos I of Theoretical and Computational Physics (iC1), University of Granada（格拉纳达大学理论计算物理卡洛斯一世研究所）；University CEU Fernando III（塞乌费尔南多三世大学）；Higher Polytechnic School, University CEU Fernando III（塞乌费尔南多三世大学高等理工学院）；University of Jaen（哈恩大学）；Department of Mathematics, University of Jaen（哈恩大学数学系）
