Forward stability conjecture for the Schur canonical form

Let A0=U0T0U0Cn×nA_0=U_0T_0U_0^*\in\mathbb{C}^{n\times n}, where U0U_0 is unitary and T0T_0 is upper triangular. A factorization of AA as A=UTUA=UTU^* is a Schur decomposition when UU is unitary and TT is upper triangular. Forward stability conjecture. There exist constants K,ϵ>0K,\epsilon>0, depending only on A0A_0, such that for every AA satisfying

AA0<ϵ,\|A-A_0\|<\epsilon,

there is a Schur decomposition A=UTUA=UTU^* for which

UU0+TT0KAA01/n.\|U-U_0\|+\|T-T_0\|\leq K\|A-A_0\|^{1/n}.

The conjecture seeks a Hölder-type forward stability bound for the Schur factors, with the exponent 1/n1/n matching the general eigenvalue stability bound. Whether such a factorization always exists with constants depending only on A0A_0 is not resolved by the supplied context.

Sources & referencesView supporting material

Primary source

Anastasiia Minenkova, Evelyn Nitch-Griffin and Vadim Olshevsky, “Backward Stability of the Schur Canonical Form”, arXiv:2108.02312 (2021).

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