Luca Aceto Zoltán Ésik Anna Ingólfsdóttir Axiomatizing Tropical Semirings. 42-56 2001 conf/fossacs/2001 FoSSaCS http://dx.doi.org/10.1007/3-540-45315-6_3 db/conf/fossacs/fossacs2001.html#AcetoEI01