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The symmetrization map and \(\Gamma\)-contractions. (English) Zbl 07792602

Summary: The symmetrization map \(\pi :\mathbb{C}^2 \rightarrow\mathbb{C}^2\) is defined by \(\pi (z_1, z_2)=(z_1 +z_2, z_1 z_2)\). The closed symmetrized bidisc \(\Gamma\) is the symmetrization of the closed unit bidisc \(\overline{\mathbb{D}^2}\), that is, \[ \Gamma = \pi (\overline{\mathbb{D}^2})=\{ (z_1 +z_2, z_1 z_2)\,:\, |z_i| \leq 1, i=1,2\}. \] A pair of commuting Hilbert space operators \((S, P)\) for which \(\Gamma\) is a spectral set is called a \(\Gamma\)-contraction. Unlike the scalars in \(\Gamma\), a \(\Gamma\)-contraction may not arise as a symmetrization of a pair of commuting contractions, even not as a symmetrization of a pair of commuting bounded operators. We characterize all \(\Gamma\)-contractions which are symmetrization of pairs of commuting contractions. We show by constructing a family of examples that even if a \(\Gamma\)-contraction \((S,P)=(T_1 +T_2, T_1 T_2)\) for a pair of commuting bounded operators \(T_1, T_2\), no real number less than 2 can be a bound for the set \(\{ \Vert T_1 \Vert, \Vert T_2 \Vert\}\) in general. Then we prove that every \(\Gamma\)-contraction \((S, P)\) is the restriction of a \(\Gamma\)-contraction \((\widetilde{S}, \widetilde{P})\) to a common reducing subspace of \(\widetilde{S}, \widetilde{P}\) and that \((\widetilde{S}, \widetilde{P})=(A_1 +A_2, A_1 A_2)\) for a pair of commuting operators \(A_1, A_2\) with \(\max \{\Vert A_1\Vert, \Vert A_2\Vert\} \leq 2\). We find new characterizations for the \(\Gamma\)-unitaries and describe the distinguished boundary of \(\Gamma\) in a different way. We also show some interplay between the fundamental operators of two \(\Gamma\)-contractions \((S, P)\) and \((S_1, P)\).

MSC:

47A13 Several-variable operator theory (spectral, Fredholm, etc.)
47A15 Invariant subspaces of linear operators
47A20 Dilations, extensions, compressions of linear operators
47A25 Spectral sets of linear operators

References:

[1] Agler, J.; Young, NJ, A commutant lifting theorem for a domain in \({\mathbb{C} }^2\) and spectral interpolation, J. Funct. Anal., 161, 452-477 (1999) · Zbl 0943.47005 · doi:10.1006/jfan.1998.3362
[2] Agler, J.; Young, NJ, Operators having the symmetrized bidisc as a spectral set, Proc. Edinb. Math. Soc. (2), 43, 195-210 (2000) · Zbl 0983.47004 · doi:10.1017/S0013091500020812
[3] Agler, J.; Young, NJ, A model theory for \(\Gamma \)-contractions, J. Oper. Theory, 49, 45-60 (2003) · Zbl 1019.47013
[4] Agler, J.; Young, NJ, The hyperbolic geometry of the symmetrized bidisc, J. Geom. Anal., 14, 375-403 (2004) · Zbl 1055.32010 · doi:10.1007/BF02922097
[5] Agler, J.; Lykova, Z.; Young, NJ, A geometric characterization of the symmetrized bidisc, J. Math. Anal. Appl., 473, 1377-1413 (2019) · Zbl 1412.53062 · doi:10.1016/j.jmaa.2019.01.027
[6] Agler, J.; Lykova, Z.; Young, NJ, Intrinsic directions, orthogonality, and distinguished geodesics in the symmetrized bidisc, J. Geom. Anal., 31, 8202-8237 (2021) · Zbl 1477.32041 · doi:10.1007/s12220-020-00582-0
[7] Bhattacharyya, T., Sau, H.: Interpolating sequences and the Toeplitz corona theorem on the symmetrized bidisk. J. Oper. Theory (to appear). arxiv:1909.03237 · Zbl 1524.46034
[8] Bhattacharyya, T.; Pal, S., A functional model for pure \(\Gamma \)-contractions, J. Oper. Thoery, 71, 327-339 (2014) · Zbl 1324.47015 · doi:10.7900/jot.2012mar21.1946
[9] Bhattacharyya, T.; Sau, H., Holomorphic functions on the symmetrized bidisk-realization, interpolation and extension, J. Funct. Anal., 274, 504-524 (2018) · Zbl 1384.32003 · doi:10.1016/j.jfa.2017.09.013
[10] Bhattacharyya, T.; Pal, S.; Roy, SS, Dilations of \(\Gamma \)- contractions by solving operator equations, Adv. Math., 230, 577-606 (2012) · Zbl 1251.47010 · doi:10.1016/j.aim.2012.02.016
[11] Bhattacharyya, T.; Das, BK; Sau, H., Toeplitz operators on the symmetrized bidisc, Int. Math. Res. Not. IMRN, 11, 8492-8520 (2021)
[12] Bhattacharyys, T.; Lata, S.; Sau, H., Admissible fundamental operators, J. Math. Anal. Appl., 425, 983-1003 (2015) · Zbl 1304.47009 · doi:10.1016/j.jmaa.2015.01.006
[13] Nikolov, N.; Pflug, P.; Thomas, PJ, Spectral Nevanlinna-Pick and Caratheodory-Fejer problems for \(n\le 3\), Indiana Univ. Math. J., 60, 883-893 (2011) · Zbl 1256.30020 · doi:10.1512/iumj.2011.60.4310
[14] Pal, S., From Stinespring dilation to Sz.-Nagy dilation on the symmetrized bidisc and operator models, N. Y. J. Math., 20, 645-664 (2014) · Zbl 1314.47016
[15] Pal, S.; Shalit, OM, Spectral sets and distinguished varieties in the symmetrized bidisc, J. Funct. Anal., 266, 5779-5800 (2014) · Zbl 1311.47008 · doi:10.1016/j.jfa.2013.12.022
[16] Pflug, P.; Zwonek, W., Exhausting domains of the symmetrized bidisc, Ark. Mat., 50, 397-402 (2012) · Zbl 1262.32001 · doi:10.1007/s11512-011-0153-5
[17] Sarkar, J., Operator theory on symmetrized bidisc, Indiana Univer. Math. J., 64, 847-873 (2015) · Zbl 1330.47016 · doi:10.1512/iumj.2015.64.5541
[18] Sz.-Nagy, B., Foias, C., Kerchy, L., Bercovici, H.: Harmonic Analysis of Operators on Hilbert Space. Universitext, Springer, New York (2010) · Zbl 1234.47001
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