Toughness for Fractional (2, b, k)-Critical Covered Graphs

Let h:E(G)→[0,1]be a function.If a≤∑e∋xh(e)≤b holds for each x∈V(G),then we call G[Fh]a fractional[a,b]-factor of G with indicator function h,where Fh={e:e∈E(G),h(e)>0}.A graph G is called a fractional[a,b]-covered graph if for every edge e of G,there is a fractional[a,b]-factor G[Fh]with h(e)=1.Zhou,Xu and Sun[S.Zhou,Y.Xu,Z.Sun,Degree conditions for fractional(a,b,k)-critical covered graphs,Information Processing Letters 152(2019)105838]defined the concept of a fractional(a,b,k)-critical covered graph,i.e.,for every vertex subset Q with|Q|=k of G,G−Q is a fractional[a,b]-covered graph.In this article,we study the problem of a fractional(2,b,k)-critical covered graph,and verify that a graph G withδ(G)≥3+k is a fractional(2,b,k)-critical covered graph if its toughness t(G)≥1+1b+k2b,where b and k are two nonnegative integers with b≥2+k2.