Differential equations 4 Existence of positive periodic solutions Problem solving 4 A priori estimates 2 A-priori estimates 2 Angiogenesis 2 Cell physiology 2 Complex model 2 Cytology 2 Delay differential equations 2 Differentiation (calculus) 2 LeraySchauder degree methods 2 Mathematical models 2 Models with feedback 2 Nonautonomous 2 Nonlinear equations 2 Nonlinear nonautonomous delay differential equations 2 Periodic solution 2 Periodic solutions 2 Physiology 2 Regulatory mechanism 2 Second order Liénard type equation 2 Second orders 2 Self regulation 2 Systems of DDEs 2 Topological degree theory 2 Topology 2