@bookEndres-Fröhlich_Hehenkamp_Heinzel, title=The Impact of Product Differentiation on Retail Bundling in a Vertical Market, author=Endres-Fröhlich, Angelika Elfriede and Hehenkamp, Burkhard and Heinzel, Joachim 

@bookHeinzel_2019, place=Paderborn University, title=Bundling in a Distribution Channel with Retail Competition, publisher=CIE Working Paper Series, author=Heinzel, Joachim Maria Josef, year=2019