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For a graph \( G \) with vertex set \( V(G) \) and edge set \( E(G) \), let \( i(G) \) be the number of isolated vertices in \( G \). The \emph{isolated toughness} of \( G \) is defined as
\[
I(G) = \min\left\{\frac{|S|}{i(G-S)} \mid S \subseteq V(G), i(G-S) \geq 2 \right\},
\]
if \( G \) is not complete; and \( I(K_n) = n-1 \). In this paper, we investigate the existence of \([a, b]\)-factors in terms of this graph invariant. We proved that if \( G \) is a graph with \( \delta(G) \geq a \) and \( I(G) \geq a \), then \( G \) has a fractional \( a \)-factor. Moreover, if \( \delta(G) \geq a \), \( I(G) > (a-1) + \frac{a-1}{b} \), and \( G-S \) has no \( (a-1) \)-regular component for any subset \( S \) of \( V(G) \), then \( G \) has an \([a, b]\)-factor. The latter result is a generalization of Katerinis’ well-known theorem about \([a, b]\)-factors (P. Katerinis, Toughness of graphs and the existence of factors, \emph{Discrete Math}. 80(1990), 81-92).