A characterization of compact operators on $\ell^p$-spaces
Let $A$ be a Banach space, $p>1$, and $1/p+1/q=1$. If a sequence $a=(a_i)$ in $A$ has a finite $p$-sum, then the operator $Λ_a:\ell^q\to A$, defined by $Λ_a(β)=\sum_{i=1}^\infty β_i a_i, β=(β_i)\in \ell^q$, is compact. We present a characterization of compact operators $Λ:\ell^q\to A$, and prove that $Λ$ is compact if and only if $Λ=Λ_a$, for some sequence $a=(a_i)$ in $A$ with $\{(ϕ(a_i)): ϕ\in A^*, \|ϕ\|\leq 1\}$ being a totally bounded set in $\ell^p$. For a sequence $(T_i)$ of bounded operators on a Hilbert space $H$, the corresponding operator $T:\ell^q\to B(H)$, defined by $T(β) = \sum_{i=1}^\infty β_i T_i$, is compact if and only if the set $\{\langle T x,x\rangle:\|x\|=1\}$ is a totally bounded subset of $\ell^p$, where $\langle T x,x\rangle = (\langle T_1 x,x\rangle, \langle T_2 x,x\rangel, \dotsc)$, for $x\in H$. Similar results are established for $p=1$ and $p=\infty$.