Suppose that X is Banach space. How to prove that the center of algebra B(X)( bounded operators on X) consists only of operators aI, where a is scalar and I is identity operator?
Maybe this is hw ? Anyway, you always have rank one operators $x\mapsto L(x)v$, $v\in X$, $L\in X^*$, and any operator which commutes with all of them is easily seen to be scalar. 


The argument of BS works also in the case where $X$ is a Hausdorff locally convex space since the topological dual still separates points (by HahnBanach). This is enough to show that the (continuous) finite rank operators act transitively on nonzero vectors from which it follows that their center is already trivial. But then all bounded operators have at most the center of the finite rank ones and the multiples of the identity. So if you want to go beyond trivial center you have to consider topological vector spaces which are not lc, pretty weird ones ;) I don't know the answer for $L^p$ spaces with $p < 1$...? I guess the more interesting problem is whether there are more bounded operators than the continuous finite rank operators and the multiples of the identity... 


If I see it correctly, then take a Hamel basis $(v_i)$ with $i \in I$ of the Banach space $X$. Then the coordinate switch $T_{i,j}:v_k \mapsto \delta_{i,k}v_j$ is bounded, because finite dimensional subspaces have a complement in a Banach space. Writing down any $S$ in the centre of $\mathscr{B}(X)$ as a matrix with rows and columns indexed by $I$, commutativity of $S$ and $T_{i,i}$ shows that is must be diagonal and commutativity of $S$ and $T_{i,j}$ shows that all diagonal entries must be the same. Hence $S$ is a scalar multiple of the identity. The essence of the argument is that finite dimensional subspaces are complemented in Banach spaces. 

