function ydot=RHS_SIPF(t,y)


%  RHS_SIP
%
%    define the RHS of the model equations. 
%        variables are 
%  y(1): S, susceptible, uninfected individuals [number]
%  y(2): I, infected individuals [number]
%  y(3): P, infectious particles close 
%            to the susceptible population [number]
%  y(4): F, total infectious particles in the susceptible population (absorved or filtered)
%  population 
%            
%   array indexes of variables

nVar=4;iS=1;iI=2;iP=3;iF=4;

global PAR 

% most rates are 1/day

ydot=zeros(nVar,1);


% EQUATIONS 

% Time changes of susceptable individuals,

ydot(iS)= - PAR.Finfect * y(iF);


% Time changes of susceptable individuals,

ydot(iI)=  PAR.Finfect * y(iF)- PAR.Imort * y(iI);

% Time changes of susceptable individuals,

ydot(iP)=  PAR.Pburden *PAR.Prelease * y(iI)- PAR.Premove * y(iP)- PAR.Pfilt*y(iS)*y(iP)- PAR.Pfilt*y(iI)*y(iP);

% Time changes of total particle in  the S popualtion 

ydot(iF)=  PAR.Pfilt *y(iP) * y(iS)- PAR.Predin * y(iF);