DOI: 10.68381/jca21045 ISSN: 0944-6532

Measures of Weak Noncompactness in Non-Archimedean Banach Spaces

Carlos Angosto, Jerzy Kąkol, Albert Kubzdela

Let

E E
be a non-Archimedean Banach space over a non-Archimedean locally compact non-trivially valued field
\mathbb{K}:=(\mathbb{K},|.|) K : = ( K , ∣ . ∣ )
. Let
E'' E ′ ′
be its bidual and
M M
a bounded set in
E E
. We say that
M M
is
\varepsilon ε
-weakly relatively compact if
\ \overline{M}^{\sigma (E'',E')}\subset E+B_{E^{\prime \prime },\varepsilon}   M ‾ σ ( E ′ ′ , E ′ ) ⊂ E + B E ′ ′ , ε
, where
B_{E^{\prime \prime },\varepsilon } B E ′ ′ , ε
is the closed ball in
E'' E ′ ′
with the radius
\varepsilon \geq 0 ε ≥ 0
. In this paper we describe measures of noncompactness
\gamma, γ ,
k k
and De Blasi measure
\omega ω
. We show that
\gamma \left( M\right) \leq k\left( M\right) \leq \omega \left( M\right) =\omega (acoM)\leq \frac{1}{\left\vert \rho \right\vert }\gamma \left( M\right), γ ( M ) ≤ k ( M ) ≤ ω ( M ) = ω ( a c o M ) ≤ 1 ∣ ρ ∣ γ ( M ) ,
where
\rho ρ
(
\left\vert \rho \right\vert <1) ∣ ρ ∣ < 1 )
is an uniformizing element in
\mathbb{K} K
, and
\omega (M)=\sup \{\overline{\lim_{m}}\,\,\,dist\left( x_{m},\left[ x_{1},\dots,x_{m-1}\right] \right):\left( x_{m}\right) \subset M ω ( M ) = sup ⁡ { lim ⁡ m ‾       d i s t ( x m , [ x 1 , … , x m − 1 ] ) : ( x m ) ⊂ M
\} }
; the latter equality is purely non-Archimedean. In particular, assuming
\left\vert \mathbb{K}\right\vert =\{||x||:x\in E\}, ∣ K ∣ = { ∣ ∣ x ∣ ∣ : x ∈ E } ,
we prove that the absolutely convex hull
acoM a c o M
of a
\varepsilon - ε −
weakly relatively compact subset
M M
in
E E
is
\varepsilon - ε −
weakly relatively compact. In fact we show that in this case for a bounded set
M M
in
E E
we have
\gamma \left( M\right) =\gamma \left( acoM\right) =k\left( M\right) =k(acoM)=\omega \left( M\right) γ ( M ) = γ ( a c o M ) = k ( M ) = k ( a c o M ) = ω ( M )
, Note that the above equalities fail in general for real Banach spaces by results of A. S. Granero [An extension of the Krein-Smulian theorem, Rev. Mat. Iberoam. 22 (2006) 93–100] and K. Astala and H. O. Tylli [Seminorms related to weak compactness and to Tauberian operators, Math. Proc. Cambridge Philos. Soc. 107 (1990) 367–375]. Most proofs are strictly non-Archimedean. A non-Archimedean variant of another quantitative Krein's theorem due to Fabian, Hajek, Montesinos and Zizler is also provided, see Corollary 9.