@article{iiasa4323,
           title = {The arithmetics of mutual help},
       publisher = {NPG},
            year = {1995},
           pages = {50--55},
         journal = {Scientific American},
             url = {https://pure.iiasa.ac.at/id/eprint/4323/},
            issn = {1946-7087},
          author = {Sigmund, K. and Nowak, M. A. and May, R. M.}
}