Fig 2. Schematic diagram of the time evolution of POP coefficients α _t(k) with an initial value α ₀ = (α ^r, α ⁱ) = (0,1) (From [19]).

The complex number α _t rotates in slightly more than eight time steps anticlockwise once around the origin. ω is a constant angle in the complex plane. The e-folding time τ, for which |α _τ| = 1/e is marked by an open circle. The amplitude |α _t|, in the perspective of epidemiology, corresponds to magnitude of schistosomiasis risk in our study.