ar X iv : 0 90 5 . 27 40 v 1 [ m at h . C O ] 1 7 M ay 2 00 9 Graphs with many ± 1 or ± √ 2 eigenvalues
نویسنده
چکیده
A pseudo (v, k, λ)-design is a pair (X,B) where X is a v-set and B = {B1, . . . , Bv−1} is a collection of k-subsets (blocks) of X such that each two distinct Bi, Bj intersect in λ elements; and 0 < λ < k < v − 1. We use the notion of pseudo designs to characterize graphs of order n whose spectrum contains either ±1 or ± √ 2 with multiplicity (n − 2)/2 or (n− 3)/2. It turns out that the subdivision of the star K1,k is determined by its spectrum if k 6∈ {l − 1 | l ∈ N} ∪ {l − l | l ∈ N}. Meanwhile, partial results confirming a conjecture of O. Marrero on characterization of pseudo (v, k, λ)-designs are obtained. AMS Classification: 05C50; 05B05; 05B30
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تاریخ انتشار 2009