$\begin{array}{ll}
f(x)=\sec ^{-1}(2[x]+1) \
\Rightarrow 2[x]+1 \geq 1 & \text { or } 2[x]+1 \leq-1 \
\Rightarrow 2[x] \geq 0 & \text { or } 2[x] \leq-2 \
\Rightarrow[x] \geq 0 & \text { or }[x] \leq-1 \
\Rightarrow x \geq 0 & \text { or } x \leq 0
\end{array}Domainoff(x)is(-\infty, \infty)$