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<q^{2}} = √ 2.

But more commonly, you will find it written as "inf

_{some 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.