This is a simple mathematical model of intracellularcircadian clock developed in the paper
Jae Kyoung Kim and Daniel B. Forger (2012) "A mechanism for robust circadian timekeeping via balanced stoichiometry", Molecular Systems Biology
SNF: Single negative feedback loop.
NNF: Negative feedback loop with additional negative feedback loop.
PNF: Negative feedback loop with additional positive feedback loop.
The model is available in the format of
1) XPPAUT: *.ode
2) Matlab: *.m
3) Mathematica: *.nb
4) SBML: *.xml