@ARTICLE{
author={Ivkovic Ivan S,Janjos Zeljko,Zezelj Srecko,Jovanovic Vojkan D},
year={2011},
title={Research into the costs of vehicle exhaust gases from the standpoint of using natural gas-powered buses},
journal={AFRICAN JOURNAL OF BUSINESS MANAGEMENT},
volume={5},
number={22},
pages={9304-9321},
document_type={Article},
} 

@ARTICLE{
author={Petrovic Jelica M,Ivkovic Ivan S,Vujacic Ivan,Zezelj Srecko},
year={2009},
title={Possibilities of Buses on Alternative Fuel in Public Urban Transport in Belgrade},
journal={TECHNOLOGICAL AND ECONOMIC DEVELOPMENT OF ECONOMY},
volume={15},
number={1},
pages={78-89},
document_type={Article},
}