Abstract
An open problem posed by G. Maksa and Z. Páles is to find the general solution of the functional equation
where \(f: H \rightarrow K\) is between two complex normed spaces and \({\mathbb {T}}_n:=\{e^{i\frac{2k\pi }{n}}: k=1, \cdots ,n\}\) is the set of the nth roots of unity. With the aid of the celebrated Wigner’s unitary-antiunitary theorem, we show that if \(n\ge 3\) and H and K are complex inner product spaces, then f satisfies the above equation if and only if there exists a phase function \(\sigma : H\rightarrow {\mathbb {T}}_n\) such that \(\sigma \cdot f\) is a linear or anti-linear isometry. Moreover, if the solution f is continuous, then f is a linear or anti-linear isometry.
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The first and second authors were supported by the Natural Science Foundation of Tianjin City (Grant No. 22JCYBJC00420) and the National Natural Science Foundation of China (Grant No. 12271402). The third author was supported by the National Natural Science Foundation of China (Grant Nos. 12201459 and 12071358) and is the corresponding author.
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Huang, X., Zhang, L. & Wang, S. A functional equation related to Wigner’s theorem. Aequat. Math. (2024). https://doi.org/10.1007/s00010-024-01042-8
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DOI: https://doi.org/10.1007/s00010-024-01042-8