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Renorming of \(\ell _{1}\) and the fixed point property. (English) Zbl 1187.46005

In [Nonlinear Anal. 68, 2303–2308 (2008; Zbl 1151.46006)], the author proved that \((\ell_1,\|\cdot\|_1)\) could be renormed with an equivalent norm \(|||\cdot|||\) so that \((\ell_1, |||\cdot|||)\) has the fixed point property for nonexpansive self-mappings of closed, bounded, convex subsets of \(\ell_1\). In this article, the author uses the ideas introduced in the previous article to prove that a class of renormings of \(\ell_1\) have the fixed point property. In particular, for \(k\in{\mathbb{N}}\), let \(P_k\) denote the natural projections on \(\ell_1\) and let \(|||\cdot|||\) be an equivalent norm on \(\ell_1\) satisfying the following four conditions: 6mm
(1)
The unit ball of \((\ell_1, |||\cdot|||)\) is \(\sigma(\ell_1,c_0)\)-closed.
(2)
For any \(k\in{\mathbb{N}}\), \(\| I-P_k \| = 1\).
(3)
There exist \(\alpha>4\) and a decreasing sequence \((\alpha_k)\) in \((0,1)\) such that, for any normalized block basis \((f_n)\) of \((\ell_1, |||\cdot|||)\) and \(x\in\ell_1\) with \(P_{k-1}(x)=x\) and \(|||x|||<\alpha_k\), \(\limsup_n |||f_n+x||| \leq 1 + |||x|||/\alpha\).
(4)
There exist strictly decreasing sequences \((\beta_k)\) and \((\gamma_k)\) with \(\lim_k \beta_k = 0\) and \(\lim_k \gamma_k = 1\) such that, for any normalized block basis \((f_n)\) of \((\ell_1, |||\cdot|||)\) and any \(x\in\ell_1\) with \(P_{k-1}(x)=x\), \(\liminf_n |||f_n+x|||\geq 1 - \beta_k + |||x|||/\gamma_k\).
Then \((\ell_1, |||\cdot|||)\) has the fixed point property.

MSC:

46B03 Isomorphic theory (including renorming) of Banach spaces
47H10 Fixed-point theorems

Citations:

Zbl 1151.46006
Full Text: DOI

References:

[1] Dowling, P. N.; Lennard, C. J.; Turett, B., Renormings of \(\ell_1\) and \(c_0\) and fixed point properties, (Handbook of Metric Fixed Point Theory (2001), Kluwer Acad. Publ.: Kluwer Acad. Publ. Dordrecht), 269-297 · Zbl 1026.47037
[2] Goebel, K.; Kirk, W. A., Topics in Metric Fixed Point Theory (1990), Cambridge Univ. Press: Cambridge Univ. Press Cambridge · Zbl 0708.47031
[3] Kirk, W. A.; Sims, B., Handbook of Metric Fixed Point Theory (2001), Kluwer Acad. Publ.: Kluwer Acad. Publ. Dordrecht · Zbl 0970.54001
[4] Lin, P. K., There is an equivalent norm on \(\ell_1\) that has the fixed point property, Nonlinear Anal., 68, 2303-2308 (2008) · Zbl 1151.46006
[5] Maurey, B., Point fixes des contractions de centains faiblement compacts de \(L^1\), (Séminaire d’Analyse Fonctionelle (Paris) (1981), École Polytechnique: École Polytechnique Paris) · Zbl 0476.46023
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