Table 1 Relation between geometry of the nullclines and functionality

+

S > ϕ_{X = 0}> ξ

P ^-

ξ > S > ϕ_{X = 0}

G ⁺

-

S > ϕ_{X = 0}> ξ

G ^-

ξ > S > ϕ_{X = 0}

G⁺ if Z _f > Z _i else impossible

+

ϕ_{X = 0}> ξ > S

P ^-

ξ > ϕ_{X = 0}> S

G ⁺

-

ϕ_{X = 0}> ξ > S

G ^-

ξ > ϕ_{X = 0}> S

P ⁺

+

ϕ_{X = 0}> S > ξ

G^- if Z _f <Z _i else impossible

S > ξ > ϕ_{X = 0}

G ⁺

-

ϕ_{X = 0}> S > ξ

G ^-

S > ξ > ϕ_{X = 0}

P ⁺