The SPD P_st(x) as a function of x when m ≤ 0 ; (a) r = 0.1, m = −1, K = 10, P = 0.6, D = 0.5, τ₂ = 0.5 are fixed, τ₁ varies from 0.5, 5 to 50; (b) r = 0.1, m = −1, K = 10, P = 0.3, D = 0.5, τ₁ = 0.2 are fixed, τ₂ varies from 0.5, 5 to 50.