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    Extremal incomplete sets in finite abelian groups

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    Let GG be a finite abelian group. The critical number cr(G){\rm cr}(G) of GG is the least positive integer β„“\ell such that every subset AβŠ†Gβˆ–{0}A\subseteq G\setminus\{0\} of cardinality at least β„“\ell spans GG, i.e., every element of GG can be written as a nonempty sum of distinct elements of AA. The exact values of the critical number have been completely determined recently for all finite abelian groups. The structure of these sets of cardinality cr(G)βˆ’1{\rm cr}(G)-1 which fail to span GG has also been characterized except for the case that ∣G∣|G| is an even number and the case that ∣G∣=pq|G|=pq with p,qp,q are primes. In this paper, we characterize these extremal subsets for ∣G∣β‰₯36|G|\geq 36 is an even number, or ∣G∣=pq|G|=pq with p,qp,q are primes and qβ‰₯2p+3q\geq 2p+3.Comment: Swith some notations into ones which are more popular and put the materials of Appendix into the body. The present version is to appear in Ars Combinatori
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