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Pretty good state transfer on circulant graphs. (English) Zbl 1364.05027
Summary: Let $$G$$ be a graph with adjacency matrix $$A$$. The transition matrix of $$G$$ relative to $$A$$ is defined by $$H(t):=\exp(-itA)$$, where $$t\in\mathbb R$$. The graph $$G$$ is said to admit pretty good state transfer between a pair of vertices $$u$$ and $$v$$ if there exists a sequence of real numbers $$\{t_k\}$$ and a complex number $$\gamma$$ of unit modulus such that $$\lim_{k\to\infty} H(t_k)e_u=\gamma e_v.$$ We find that the cycle $$C_n$$ as well as its complement $$\overline{C}_n$$ admit pretty good state transfer if and only if $$n$$ is a power of two, and it occurs between every pair of antipodal vertices. In addition, we look for pretty good state transfer in more general circulant graphs. We prove that union (edge disjoint) of an integral circulant graph with a cycle, each on $$2^k$$ $$(k\geq 3)$$ vertices, admits pretty good state transfer. The complement of such union also admits pretty good state transfer. Using Cartesian products, we find some non-circulant graphs admitting pretty good state transfer.

##### MSC:
 05C12 Distance in graphs 05C50 Graphs and linear algebra (matrices, eigenvalues, etc.)
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