# Final size for SIR model

function final_size=SIR_fs(N,bet,gamm)
final_size = zeros(N+1,1);
final_size(2) = 1;
for Z2 = 0:N;
for Z1 = Z2+1:N-1
p1 = 1 / ( 1 + gamm/(bet*(N-Z1)));
final_size(Z1+2) = final_size(Z1+2) + final_size(Z1+1)*p1;
final_size(Z1+1) = final_size(Z1+1)*(1-p1);
end
end
endfunction

N = 20;
bet = 2/(N-1);
gamm = 1;

id = tic();
final_size=SIR_fs(N,bet,gamm);
toc(id)

Elapsed time is 0.0100751 seconds.


bar(0:N,final_size)


## Final size for SI(4)R model

function final_size=SI4R_fs(N,bet,gamm)
Psi = (N+1)*(N+2)*(N+3)*(N+4)/24;
final_size = zeros(N+1,1);
p_vec = zeros(Psi,1);
p_vec(2)=1;

for Z5 = 0:N
w = Psi - (N-Z5+1)*(N-Z5+2)*(N-Z5+3)*(N-Z5+4)/24 + 1;
for Z4 = Z5:N
a5 = 4*gamm*(Z4-Z5);
for Z3 = Z4:N
a4 = 4*gamm*(Z3-Z4);
for Z2 = Z3:N;
a3 = 4*gamm*(Z2-Z3);
for Z1 = Z2:N
a1 = bet*(N-Z1)*(Z1-Z5);
a2 = 4*gamm*(Z1-Z2);
tot = a1+a2+a3+a4+a5;
if Z1-Z5 == 0
final_size(Z5+1) = p_vec(w);
end
if a1 > 0
p_vec(w+1) = p_vec(w+1)+ p_vec(w)*a1/tot;
end
if a2 > 0
p_vec(w+N-Z2) = p_vec(w+N-Z2)+ p_vec(w)*a2/tot;
end
if a3 > 0
place3 = (N-Z3)*(N-Z3+1)/2;
p_vec(w+place3) = p_vec(w+place3)+ p_vec(w)*a3/tot;
end
if a4 > 0
place4 = (N-Z4)*(N-Z4+1)*(N-Z4+2)/6;
p_vec(w+place4) = p_vec(w+place4) + p_vec(w)*a4/tot;
end
if a5 > 0
p_vec(w) = p_vec(w)*a5/tot;
end
w = w + 1;
end
end
end
end
end
endfunction

id = tic();
final_size=SI4R_fs(N,bet,gamm);
toc(id)

Elapsed time is 3.31001 seconds.


bar(0:N,final_size)