Reverse Lexicographic and Lexicographic Shifting

A short new proof of the fact that all shifted complexes are fixed by reverse lexicographic shifting is given. A notion of lexicographic shifting, $\Delta_{\lex}$ -- an operation that transforms a monomial ideal of $S=\field[x_i: i\in\N]$ that is finitely generated in each degree into a squarefree strongly stable ideal -- is defined and studied. It is proved that (in contrast to the reverse lexicographic case) a squarefree strongly stable ideal $I\subset S$ is fixed by lexicographic shifting if and only if $I$ is a universal squarefree lexsegment ideal (abbreviated USLI) of $S$. Moreover, in the case when $I$ is finitely generated and is not a USLI, it is verified that all the ideals in the sequence $\{\Delta_{\lex}^i(I)\}_{i=0}^{\infty}$ are distinct. The limit ideal $\bar{\Delta}(I)=\lim_{i\to\infty}\Delta_{\lex}^i(I)$ is well defined and is a USLI that depends only on a certain analog of the Hilbert function of $I$.