(OverHat[β])=arg (min_β)(1/2)(Subscript[\[LeftDoubleBracketingBar]l-(Subscript[D, l])·β\[RightDoubleBracketingBar], 2]^2)      s.t.   (Subscript[\[LeftDoubleBracketingBar]β\[RightDoubleBracketingBar], 1])<k