# If the cardinality of $B(X)$, the space of operators on $X$, is continuum, must $X$ be separable?

Does there exits any non-separable Banach space $X$ such that the size (cardinal number) of $B(X)$, bdd linear operators on $X$, is just of the continuum?

• What is $B(X)$? – Taras Banakh Jun 7 '18 at 18:51
• @TarasBanakh: Edited. – Ali Bagheri Jun 7 '18 at 19:03
• I hope that a counterexample can be found in papers of Koszmider, for example, in impan.pl/~koszmider/badania/fewsur2.pdf or link.springer.com/article/10.5052%2FRACSAM.2010.19 – Taras Banakh Jun 7 '18 at 19:19
• @TarasBanakh, the problem with spaces $K$ such that every operator on $C(K)$ is of the form $gI+W$ where $g\in C(K)$ and $W$ weakly compact, is that there is no clear way how to count weakly compact operators on them. – Tomasz Kania Jun 8 '18 at 21:47
• @TomekKania The number of weakly compact operators is the number of weakly compact sets times the number of continuous operators mapping the unit ball to a given weakly compact set. The number of weakly compact sets should not exceed $|X^*|^\omega$ and if all weakly compact sets are metrizable (in the weak topology), then the number of continuous operators mapping the unit ball to a given weakly compact set also should not exceed $|X^*|^\omega$. So, for a Banach space $X$ with small'' weakly compact sets, the number of weakly compact operators should not exceed $|X^*|^\omega$. Right? – Taras Banakh Jun 9 '18 at 6:54

An example of a non-separable Banach space $X$ with $|B(X)|=\mathfrak c$ is any non-separable Banach space $X$ whose dual $X^*$ is $w^*$-separable and has cardinality $|X^*|=\mathfrak c$.
This follows from the observation that the map $B(X)\to B(X^*)$, $T\mapsto T^*$, is injective and hence for a countable $w^*$-dense set $D$ in $X^*$ we have $$|B(X)|\le |B(X^*)|\le |(X^*)^{D}|=|X^*|^\omega=\mathfrak c^\omega=\mathfrak c.$$
A ZFC-example of a non-separable Banach space $X$ whose dual space $X^*$ is $w^*$-separable and has cardinality $|X^*|=\mathfrak c$ is the Banach space $X=C(K)$ of continuous functions on the Alexandroff two-arrow space $K$.
The $w^*$-separability of the dual space $X^*$ was proved by Corson, see Theorem 12.43 in this book. The equality $|X^*|=\mathfrak c$ can be seen analyzing the structure of (probability) measures on the compact space $K$.
Assume Martin's axiom and the negation of CH. Then $2^{\omega_1}=\mathfrak c$. Let $X=\ell_2(\omega_1)$. Every operator on $X$ is determined by its values on a dense set of cardinality $\omega_1$, hence there are at most $$|\ell_2(\omega_1)|^{\omega_1} = (\omega_1^\omega)^{\omega_1}\leqslant (2^\omega)^{\omega_1}=2^{\omega_1}=\mathfrak c$$ operators on $X$. Consequently, $|B(\ell_2(\omega_1))|=\mathfrak{c}$.