As the third form is the only form that allows simultaneous (1) preservation of the sequence of increasing atomic numbers, (2) a 14-element-wide f-block, and (3) avoidance of the split in the d-block, it has been suggested by a 2021 IUPAC preliminary report as the preferred form. Such a modification, treating Lu as a transition element rather than as an inner transition element, was first suggested by Soviet physicists Lev Landau and Evgeny Lifshitz in 1948. Following this, it was then suggested by many other physicists and chemists, and was generally the classification adopted by those who considered the issue, but textbooks generally lagged in adopting it.