We determine which Krein parameters of nonbipartite antipodal distance-regular graphs of diameter 3 and 4 can vanish, and give combinatorial interpretations of their vanishing. We also study tight distance-regular graphs of diameter 3 and 4. In the case of diameter 3, tight graphs are precisely the Taylor graphs. In the case of antipodal distance-regular graphs of diameter 4, tight graphs are precisely the graphs for which the Krein parameter q(11)(4) vanishes. (C) 2002 Elsevier Science B.V. All rights reserved.
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Univ KwaZulu Natal, Sch Math Stat & Comp Sci, ZA-4000 Durban, South AfricaUniv KwaZulu Natal, Sch Math Stat & Comp Sci, ZA-4000 Durban, South Africa
Ali, Patrick
Mukwembi, Simon
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Univ KwaZulu Natal, Sch Math Stat & Comp Sci, ZA-4000 Durban, South AfricaUniv KwaZulu Natal, Sch Math Stat & Comp Sci, ZA-4000 Durban, South Africa
Mukwembi, Simon
Dankelmann, Peter
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Univ Johannesburg, Dept Pure & Appl Math, ZA-2006 Auckland Pk, South AfricaUniv KwaZulu Natal, Sch Math Stat & Comp Sci, ZA-4000 Durban, South Africa
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Slovak Tech Univ Bratislava, Fac Civil Engn, Dept Math, Bratislava 81368, SlovakiaSlovak Tech Univ Bratislava, Fac Civil Engn, Dept Math, Bratislava 81368, Slovakia
Knor, Martin
Zhou, Sanming
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Univ Melbourne, Dept Math & Stat, Parkville, Vic 3010, AustraliaSlovak Tech Univ Bratislava, Fac Civil Engn, Dept Math, Bratislava 81368, Slovakia