\operatorname - Similar to \DeclareMathOperator, but rather than defining a macro, it produces an instance of an operator like \lim.
{ \operatorname}
\operatorname command is similar to \DeclareMathOperator, but rather than defining a macro, it produces an instance of an operator like \lim.
\operatorname{myFct}(x)
\operatorname*{myFct}_a^b(x)