# Compact Self Adjoint operator on a Hilbert Space

Let $T$ be a continuous and bounded self-adjoint compact operator on a Hilbert space H.

I want to prove that if $T^2=0$, then $T=0$.

Is there any thing wrong with the following:

$T^2$ = $TT^*=0$ impiles that all of T's eigenvalues are zero, and so as T is a compact operator and therefore has finite rank, all of T's eigenvalues are zero, and so $T=0$.

• Compact operators need not have finite rank. They only need to be the limit of finite rank operators in the operator norm. – RKD May 12 '12 at 12:56

However, there is a much simpler argument that doesn't need the spectral theorem: for any vector $x$, note that $\| Tx \|^2 = \langle Tx, Tx \rangle$. Now use the fact that $T=T^*$...
• Somewhat tangentially, naively it seems to me that in any normed space, a continuous operator satisfying $T^2=0$ should have $T=0.$ Can you give me a counterexample? I haven't been able to think of one. – Ragib Zaman May 12 '12 at 13:17
• @RagibZaman: $\begin{pmatrix} 0 & 1 \\ 0 & 0 \end{pmatrix}$ – Nate Eldredge May 12 '12 at 13:24