15.對于定義域為R的函數(shù)f(x),給出下列命題:
①若函數(shù)f(x)滿足條件f(x-1)+f(1-x)=2,則函數(shù)f(x)的圖象關(guān)于點(0,1)對稱;
②若函數(shù)f(x)滿足條件f(x-1)=f(1-x),則函數(shù)f(x)的圖象關(guān)于y軸對稱;
③在同一坐標系中,函數(shù)y=f(x-1)與y=f(1-x)其圖象關(guān)于直線x=1對稱;
④在同一坐標系中,函數(shù)y=f(1+x)與y=f(1-x)其圖象關(guān)于y軸對稱.
其中,真命題的序號是
.