On the symmetric doubly stochastic matrices that are determined by their spectra

A symmetric doubly stochastic matrix A is said to be determined by its spectra if the only symmetric doubly stochastic matrices that are similar to A are of the form $P^TAP$ for some permutation matrix P. The problem of characterizing such matrices is considered here. An almost the same but a more difficult problem was proposed by [ M. Fang, A note on the inverse eigenvalue problem for symmetric doubly stochastic matrices, Lin. Alg. Appl., 432 (2010) 2925-2927] as follows: Characterize all n-tuples $λ= (1,λ_2,...,λ_n)$ such that up to a permutation similarity, there exists a unique symmetric doubly stochastic matrix with spectrum $λ.$ In this short note, some general results concerning our two problems are first obtained. Then, we completely solve these two problems for the case n = 3. Some connections with spectral graph theory are then studied. Finally, concerning the general case, two open questions are posed and a conjecture is introduced.

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