The fitness of particles with different degrees during the optimization process.
Here , where Nk is the number of particles with degree k. And δ(ki, k) = 1 if ki = k and 0 otherwise. (a) SFPSO, i.e. kc = kmax; (b)SFIPSO, i.e. kc = kmin − 1. (c) SIPSO, i.e. kmin − 1 < kc < kmax, here we show the evolution of F(k) for kc = 5.