\bibitem[Sage]{sage} Stein, William, \emph{Sage: {O}pen {S}ource {M}athematical {S}oftware ({V}ersion 2.10.2)}, The Sage~Group, 2008, {\tt http://www.sagemath.org}.
Here is the bibtex entry:
@manual{sage, Key = {Sage}, Author = {William Stein}, Organization = {The Sage~Group}, Title = {{Sage}: {O}pen {S}ource {M}athematical {S}oftware ({V}ersion 2.10.2)}, Note= {{\tt http://www.sagemath.org}}, Year = 2008 }
If you happen to use the Sage interface to PARI, GAP or Singular, you should definitely reference them as well. Likewise, if you use code that is implemented using PARI, GAP, or Singular, reference the corresponding system (you can often tell from the Sage documentation if PARI, GAP, or Singular is used in the implementation of a function).
@manual{PARI2, organization = "{The PARI~Group}", title = "{PARI/GP, version {\tt 2.1.5}}", year = 2004, address = "Bordeaux", note = "available from \url{http://pari.math.u-bordeaux.fr/}" }
\bibitem{PARI2} PARI/GP, version {\tt 2.1.5}, Bordeaux, 2004, \url{http://pari.math.u-bordeaux.fr/}.
[GAP04] The GAP Group, GAP -- Groups, Algorithms, and Programming, Version 4.4; 2005. (http://www.gap-system.org)
or
@manual{GAP4, key = "GAP", organization = "The GAP~Group", title = "{GAP -- Groups, Algorithms, and Programming, Version 4.4}", year = 2005, note = "{\tt http://www.gap-system.org}", keywords = "groups; *; gap; manual"}
\bibitem[GAP]{GAP4} The GAP~Group, \emph{GAP -- Groups, Algorithms, and Programming, Version 4.4}; 2005, {\tt http://www.gap-system.org}.
[GPS05] G.-M. Greuel, G. Pfister, and H. Sch\"onemann. {\sc Singular} 3.0. A Computer Algebra System for Polynomial Computations. Centre for Computer Algebra, University of Kaiserslautern (2005). {\tt http://www.singular.uni-kl.de}.
@TechReport{GPS05, author = {G.-M. Greuel and G. Pfister and H. Sch\"onemann}, title = {{\sc Singular} 3.0}, type = {{A Computer Algebra System for Polynomial Computations}}, institution = {Centre for Computer Algebra}, address = {University of Kaiserslautern}, year = {2005}, note = {{\tt http://www.singular.uni-kl.de}}, }
\bibitem[GPS05]{GPS05} G.-M.~Greuel, G.~Pfister, and H.~Sch\"onemann. \newblock {{\sc Singular} 3.0}. A Computer Algebra System for Polynomial Computations. \newblock Centre for Computer Algebra, University of Kaiserslautern (2005). \newblock {\tt http://www.singular.uni-kl.de}.
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