the amsref entry should be\bib{sagemath}{manual}{author={Developers, The~Sage},title={{S}agemath, the {S}age {M}athematics {S}oftware {S}ystem({V}ersion x.y.z)},date={YYYY},note={{\tt http://www.sagemath.org}},}I have created it using the advice in https://tex.stackexchange.com/a/28123/7110(so the amsref entry would be in a .bbl file you create from .bib etc)---something that you might find useful on its own.