G.T. Cirella and  L. Tao,  The Index of Sustainable Functionality: An Application for Measuring Sustainability.   journal   = {International Journal of Humanities and Social Sciences}, [online]. World Academy of Science, Engineering and Technology.
    April 2009, vol. 28(4). 268 - 274
    [viewed 21 October 2019]. Available from: https://publications.waset.org/pdf/3464.