Published online by Cambridge University Press: 20 November 2018
Let   $L(X)$  be the space of bounded linear operators on the Banach space
 $L(X)$  be the space of bounded linear operators on the Banach space   $X$ . We study the strict singularity and cosingularity of the two-sided multiplication operators
 $X$ . We study the strict singularity and cosingularity of the two-sided multiplication operators   $S\,\mapsto \,ASB$  on
 $S\,\mapsto \,ASB$  on   $L(X)$ , where
 $L(X)$ , where   $A,\,B\,\in \,L(X)$  are fixed bounded operators and
 $A,\,B\,\in \,L(X)$  are fixed bounded operators and   $X$  is a classical Banach space. Let
 $X$  is a classical Banach space. Let   $1\,<\,p\,<\,\infty $  and
 $1\,<\,p\,<\,\infty $  and   $p\,\ne \,2$ . Our main result establishes that the multiplication
 $p\,\ne \,2$ . Our main result establishes that the multiplication   $S\,\mapsto \,ASB$  is strictly singular on
 $S\,\mapsto \,ASB$  is strictly singular on   $L\left( {{L}^{p}}\left( 0,\,1 \right) \right)$  if and only if the non-zero operators
 $L\left( {{L}^{p}}\left( 0,\,1 \right) \right)$  if and only if the non-zero operators   $A,\,B\,\in \,L\left( {{L}^{p}}\left( 0,\,1 \right) \right)$  are strictly singular. We also discuss the case where
 $A,\,B\,\in \,L\left( {{L}^{p}}\left( 0,\,1 \right) \right)$  are strictly singular. We also discuss the case where   $X$  is a
 $X$  is a   ${\mathcal{L}^{1}}-$  or a
 ${\mathcal{L}^{1}}-$  or a   ${{\mathcal{L}}^{\infty }}-$ space, as well as several other relevant examples.
 ${{\mathcal{L}}^{\infty }}-$ space, as well as several other relevant examples.