How to put a symbol above another in LaTeX?

Question

How to put a symbol above (on the head of) another? For example, I wanna produce something like this in one line.

a
#

i.e., 'a' above '#'.

EDIT:

The effect should be almost the same with $#^a# except that 'a' is on the top instead of top right of '#'.

Answer 1

Use \overset{above}{main} in math mode. In your case, \overset{a}{\#}.

Answer 2

${a \atop \#}$

or

${a \above 0pt \#}$

Answer 3

If you're using # as an operator, consider defining a new operator for it:

\newcommand{\pound}{\operatornamewithlimits{\#}}

You can then write things like \pound_{n = 1}^N and get: