\documentclass{report}

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{amssymb}

\begin{document}
	\begin{tikzpicture}
		\draw[blue] (2.75,-2) -- (2.75,3);
		\node[blue] at (3.2,3) (Komp) {$U^\perp$};
		
		\draw[thick] (0,0) -- (4,0);
		\draw[thick] (0,0) -- (1.5,1.5);
		\draw[thick] (4,0) -- (5.5,1.5);
		\draw[thick] (1.5,1.5) -- (5.5,1.5);
		\node at (3.5,0.4) (U) {$U$};
		\node at (0,2.5) (R) {$\mathbb{R}^3$};
		
		\coordinate (c2) at (2.75,0.75);
		\draw ($(c2) + (0:0.4)$) arc (0:90:0.4); % radius=4mm, initial=0, final=90
		\draw[thick] (2.90,0.90) circle (0.01);
	\end{tikzpicture}
\end{document}