## \stackrel

This can be used to stack something above something else; used in math mode.

  \stackrel{top}{bot}


The first argument, top, is typset immediately above the second argument, bot; the former is set in the same math style as a superscript.

For example

  H$_2$CO$_3$ $\stackrel{\rm heat} {\longrightarrow}$ H$_2$O + CO$_2$


will produce something like


heat
H CO   ------>  H O  +  CO
2  3            2        2


Note that the the \rm declaration is used so that "heat" is set in Roman face and not math italics.

## \mathop

This command does not seem to be well documented but appears to work, at least in current versions of LaTeX 2.09.

The argument is considered to be a single variable sized math symbol for purposes of placing limits below (subscripts) and above (superscripts) in display math style. For example,

  \mathop{\sum \sum}_{i,j=1}^{N} a_i a_j


would produce something like

            N
---  ---
\    \
/    /    a  a
---  ---   i  j
i,j


It is possible to create two rows of "subscripts" by using a similar construction: The first row is created as subscripts of the symbol in the usual way and a lower row is created by placing this whole construct as the argument of a \mathop command with its own subscript. For example

  \mathop{\sum_{i,j=1}^{N}}_{i>j}


would produce something like

          N
---
\
/
---
i,j=1
i>j


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