## Abstract

The Kuratowski measure of noncompactness a on an infinite dimensional Banach space (X,||.||) assigns to each bounded set S in X a nonnegative real number α(S) by the formula α(S) =inf{δ > 0 | S = U ^{n} _{i=1}S_{i} for some S_{i} with diam(S _{i}) ≥ δ, for 1 ≤ i ≤ n < ∞}. In general a map β which assigns to each bounded set S in X a nonnegative real number and which shares most of the properties of a is called a homogeneous measure of noncompactness or homogeneous MNC. Two homogeneous MNC's β and γ on X are called equivalent if there exist positive constants b and c with bβ(S) = γ(S) ≤ cβ(S) for all bounded sets S C X. There are many results which prove the equivalence of various homogeneous MNC's. Working with X = ℓ^{p}(N) where 1 ≤ p≤∞, we give the first examples of homogeneous MNC's which are not equivalent. Further, if X is any complex, infinite dimensional Banach space and L : X → X is a bounded linear map, one can define ?(L) = sup{|γ| | γ ε ess(L)}, where ess(L) denotes the essential spectrum of L. One can also define β(L) = inf{γ > 0 | β(LS) ≤ γβ(S) for every S ε B(X)}. The formula ρ(L) = lim m→∞ β(L^{m})1/m is known to be true if β is equivalent to a, the Kuratowski MNC; however, as we show, it is in general false for MNC's which are not equivalent to α. On the other hand, if B denotes the unit ball in X and β is any homogeneous MNC, we prove that ρ(L) = lim sup m→8 β(L^{m}B)^{1/m} = inf{γ > 0 | lim m→∞ γ^{-m}β(L^{m}B) = 0}. Our motivation for this study comes from questions concerning eigenvectors of linear and nonlinear cone-preserving maps.

Original language | English (US) |
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Pages (from-to) | 917-930 |

Number of pages | 14 |

Journal | Proceedings of the American Mathematical Society |

Volume | 139 |

Issue number | 3 |

DOIs | |

State | Published - Mar 2011 |

## All Science Journal Classification (ASJC) codes

- Applied Mathematics
- Mathematics(all)

## Keywords

- Cone map
- Essential spectral radius
- Measure of noncompactness