This paper is a survey of the [Formula: see text]-valued invariant of topological insulators used in condensed matter physics. The [Formula: see text]-valued topological invariant, which was originally called the TKNN invariant in physics, has now been fully understood as the first Chern number. The [Formula: see text] invariant is more mysterious; we will explain its equivalent descriptions from different points of view and provide the relations between them. These invariants provide the classification of topological insulators with different symmetries in which K-theory plays an important role. Moreover, we establish that both invariants are realizations of index theorems which can also be understood in terms of condensed matter physics.
Notes on topological insulators