\ | Q \ | _ {{\ mathcal {F}}}} = \ sup \ {\ vert Qf \ vert \: \ f \ in {\ mathcal {F}} \}.
\ | Q \ | _ {{\ mathcal {F}}}} = \ sup \ {\ vert Qf \ vert \: \ f \ in {\ mathcal {F}} \}.
\ | Q \ | _ {{\ mathcal {F}}}} = \ sup \ {\ vert Qf \ vert \: \ f \ in {\ mathcal {F}} \}.