s
S
1
S
2
s = S
1
/(S
1
+ S
2
) x
1
x
2
S
1
S
2
g(s) S
2
½
x
0
1
(t) = x
1
(t)(ε − β(x
1
(t) + x
2
(t)) + g(s)(x
2
(t) − x
1
(t)) , x
i
(0) = x
0
i
x
0
2
(t) = x
2
(t)(ε − β(x
1
(t) + x
2
(t)) + g(s)(x
1
(t) − x
2
(t)) − u(t) ,
(1)
x
1
(t) ≥ 0 t
x
2
(t) ≥ 0 u(t) ≥ 0
t s(t)
g(s)
J =
∞
Z
0
e
−ρt
[u(t)(p − cu(t)) − k(1 − s)]dt , (2)
c p k
I =
∞
Z
0
e
−ρt
[m(x
1
(t) + x
2
(t) − ¯x)
2
)]dt , (3)
¯x
m