“…− y)r(a, y)e −µ 0 a f 2 (t − a, y)dady, ∀t ∈ RC 21 [f 1 ](t) := β(t, •)S(t) t −∞ e −δ(t−s) f 1 (s, •)ds, ∀t ∈ R.Using similar arguments as in[13], we conclude that sign(r(C) − 1) = sign(r(C 12 • C 21 ) − 1) where the linear operator C 12 • C 21 is given by(C 12 •C 21 )[f ](t) = R N ∞ 0 K(•−y)r(a, y)e −µ 0 a β(t − a, y)S(t − a) t−a −∞ e −δ(t−a−s) f 1 (s, y)ds dady, for every f ∈ C p (R, Y ) and t ∈ R. t, s, τ ) e − s s−τ ϑ(l−s+t,l,l−s+τ )dl w(t − τ, s − τ ) dsdτ 0 threshold dynamics of (4.25) is given by the spectral radius of the following linear operator (C[w](t))(a) = diag( S(t, a)) , s, τ )e − s s−τ ϑ(l−s+t,l,l−s+τ )dl w(t − τ, s − τ )dsdτ, for all t ∈ R, m ∈ C p (R, Y ) with the notation w(t)(a) = w(t, a). Setting w = w h w m the linear operator C takes the following formC[w] = C m [w m ] C h [w h ]where we have set for k = h, m(C k [w k ](t))(a) := Sk (t, a) ∞ 0 ∞ τ β k (t, s, τ )e − s s−τ ϑ k (l−s+t,l,l−s+τ )dl w k (t − τ, s − τ )dsdτ.Using similar arguments in[13], we deduce that r(C) − 1, r(C h • C m ) − 1 and r(C m • C h ) −1have the same sign. Note that r(C h • C m ) = r(C m • C h ) is actually the basic reproduction number of (4.25)…”