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Published online by Cambridge University Press: 29 March 2010
Let a, b and n be non-negative integers such that 1 ≤ a ≤ b, and let G be a graph of order p with and f be an integer-valued function defined on V(G) such that a ≤ f(x) ≤ b for all x ∈ V(G). Let h: E(G) → [0, 1] be a function. If ∑e∋xh(e) = f(x) holds for any x ∈ V(G), then we call G[Fh] a fractional f-factor of G with indicator function h, where Fh = {e ∈ E(G): h(e) > 0}. A graph G is called a fractional (f, n)-critical graph if after deleting any n vertices of G the remaining graph of G has a fractional f-factor. In this paper, it is proved that G is a fractional (f, n)-critical graph if for every non-empty independent subset X of V(G), and . Furthermore, it is shown that the result in this paper is best possible in some sense.