## On fundamental biorthogonal systems in some conjugate Banach spaces.(Russian)Zbl 0546.46007

Let X be a Banach space. Given a fundamental biorthogonal system $$\{x_ i,X^*_ i\}_{i\in I}\subset X\times X^*$$ define $$p(\{x_ i\})=\sup_{i\in I}\| x^*_ i\|\| x_ i\|,\quad q(\{x_ i\})=\sup_{i\in I}\sup_{\| x\|\leq 1}\| x-x^*_ i(x)x_ i\|.$$ Furthermore, let $$p(X)=\inf p(\{x_ i\})$$ and $$q(X)=\inf q(\{x_ i\}),$$ where the infimum is taken over all fundamental biorthogonal systems of X. Earlier, A. Pełczyński [Stud. Math. 55, 295-304 (1976; Zbl 0336.46023)] has proved that $$p(X)=1$$ and q(X)$$\leq 2$$ for every separable Banach space X. Also, from the papers of V. F.Babenko, S. A. Pichugov [Ukr. Mat. Zh. 33, 491-492 (1981; Zbl 0492.47018)] and I. K. Daugavet [Usp. Mat. Nauk No.5(113) 157-158 (1963; Zbl 0138.386)] it is known that q(X)$$\geq 2$$ if $$X=C[0,1]$$ or $$X=L_ 1[0,1]$$. The main result of this paper is as follows:
If X is a Banach space with an unconditional orthogonal basis and its dual $$X^*$$ is nonseparable, then $$q(X^*)\geq 2$$. Particularly, many properties of the space $$\ell_{\infty}$$ are given.
Reviewer: T.Precupanu

### MSC:

 46B15 Summability and bases; functional analytic aspects of frames in Banach and Hilbert spaces 46B20 Geometry and structure of normed linear spaces 46B10 Duality and reflexivity in normed linear and Banach spaces

### Citations:

Zbl 0336.46023; Zbl 0492.47018; Zbl 0138.386
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