function [ dSdt ] = GomesMorrisPresentationfn(t,S)
global r1 rn1 r3 r2 rn2 
dSdt = zeros(size(S)); 

a = S(1);
b = S(2); 
c = S(3); 

dadt = -r1*a + rn1*b - r3*a*c 
dbdt = r1*a -rn1*b +rn2*c -r2*b +2*r3*a*c
dcdt = -rn2*c + r2*b -r3*a*c 

dSdt(1) = dadt;
dSdt(2) = dbdt;
dSdt(3) = dcdt; 
end