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Let G be a finite permutation group on. An ordered sequence (₁, _) of elements of is an irredundant base for G if the pointwise stabilizer is trivial and no point is fixed by the stabilizer of its predecessors. The minimal cardinality of a base is said to be the base size of G. If all irredundant bases of G have the same cardinality, G is said to be an IBIS group. In this paper, we classify the finite almost simple primitive IBIS groups whose base size is at least 6.
Mastrogiacomo et al. (Wed,) studied this question.