Defining new math operators

Defining a new math operator that behaves similar to \sin or \lim is very easy to do using the amsmath package. It provides a \DeclareMathOperator macro that works in the preamble to declare a new operator. It also contains a starred version that behaves similar to \lim with respect to subscripts. For example:

\DeclareMathOperator\arcsec{arcsec}
\DeclareMathOperator*\Lim{Lim}

In addition, \operatorname or \operatorname* can be used for one-time uses that don't warrant defining a new control sequence for the operator name. These are better than using \mathrm to define operator names if for no other reason than spacing is handled correctly in the presence or absence of parentheses.