In mathematics, the classifying space BSU ( n ) {\displaystyle \operatorname {BSU} (n)} for the special unitary group SU ( n ) {\displaystyle \operatorname {SU} (n)} is the base space of the universal SU ( n ) {\displaystyle \operatorname {SU} (n)} principal bundle ESU ( n ) → BSU ( n ) {\displaystyle \operatorname {ESU} (n)\rightarrow \operatorname {BSU} (n)} . This means that SU ( n ) {\displaystyle \operatorname {SU} (n)} principal bundles over a CW complex up to isomorphism are in bijection with homotopy classes of its continuous maps into BSU ( n ) {\displaystyle \operatorname {BSU} (n)} . The isomorphism is given by pullback.