As part of Ed Dubinsky's APOS theory of mathematical learning, he and his collaborators (2005) propose that students who conceive of 0.999... as a finite, indeterminate string with an infinitely small distance from 1 have "not yet constructed a complete process conception of the infinite decimal".
作为埃德·杜宾斯基的数学学习的“APOS理论”的一部分，杜宾斯基和他的合作者在2005年提出：任何一个学生，只要把0.999…设想为一个有限的、不确定的数串，与1的差是无穷小，那么他就“还没有对无限小数形成一个完整的过程概念”。