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I'm dealing with a problem related to linear transformation.
Problem: Let $f \in L\left( V \right)$, where $L\left( V \right)$ is the set of all linear operators on $V$. Prove that if $fg = gf$ for all $g \in L\left( V \right)$, then $f = ai$, where $a$ is a scalar and $i$ is identity map.
I can't go on. Is there any hint?