The SPD P_st(x) as a function of x when m ≤ 0 ; (a) r = 0.1, m = −5, K = 10, D = 0.2, τ₁ = 0.2, τ₂ = 0.5 are fixed, P takes 0.01, 0.1, 1, and 2; (b) r = 0.1, m = −5, K = 10, P = 0.5, τ₁ = 1, τ₂ = 1 are fixed, D takes 0.01, 0.1, 1, and 2.