The symbol for the infimum of a set.

The correct way to write it is in the form "inf {some set here}", for instance

inf {q∈Q : 0<q & 2<q2} = √ 2.
But more commonly, you will find it written as "infsome condition on x some function of x". For instance, the previous example would often be written as
inf q∈Q & 0<q & 2<q2 q = √ 2.