Suppose $A$ is a self adjoint operator and $B$ is a compact operator.
Can we say that $A+B$ is a self-adjoint operator?
Suppose $A$ is a self adjoint operator and $B$ is a compact operator.
Can we say that $A+B$ is a self-adjoint operator?
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No, take $A = 0$ and $B$ any compact operator which is not self-adjoint. For example $B = i \cdot \mathrm{id}$ on $M_n(\mathbb C)$.