Cristina Coppola Tiziana Pacelli Interval semimetric spaces for approximate distances. 1263-1268 2005 EUSFLAT Conf. http://www.eusflat.org/publications/proceedings/EUSFLAT-LFA_2005/papers/JEL206.pdf conf/eusflat/2005 db/conf/eusflat/eusflat2005.html#CoppolaP05