What calculators and computers say when they mean infinity. In the first case it has something to do with the display size, the latter is probably more about lazy hackers.

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 "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.