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2 clarifications

Let $X$ and $Y$ be Hilbert spaces over the real numbers (so complex conjugation plays no role, and everything will be linear in the strict sense). Let $f : X \rightarrow Y$ be a linear continuous mapping.

By the Riesz representation theorem, Hilbert spaces are isometric isomorphic to their own dual spaces. This leads to different notions of duality, which confuses me.

(i) The dual operator of $f^\ast$ is the operator $f^\ast : Y^\ast \rightarrow X^\ast$ defined by $y^\ast \mapsto ( x \mapsto y^\ast( f x ) )$

(ii) The dual operator is the adjoint, i.e. the unique operator $f^\ast : Y \rightarrow X$ such that $\forall x \in X, y \in Y : \langle fx,y \rangle_Y = \langle x,f^\ast y \rangle_X$

The transition between these two different notions seems to be is full and faithfully functiorial, to say it like that. - Nevertheless, I would like to differ between these two notions of duality; just think in analysis of the Hilbert space $H^1_0$ and its dual $H^{-1}$. But I even don't know whether think speaking about "different dualities" is not a crime.

So, I would to know whether there are different words for this, whether these are really different concepts (except whether I use the isometry or not) and how in which contexts, generally, to use these two appropiately. Although I think at least in the less algebraic parts of mathematics it would be helpful not to implicitly use the Riesz representation, this seems to be always swept under the rug.

1

# Dual operators between Hilbert spaces : With or without riesz representation

Let $X$ and $Y$ be Hilbert spaces over the real numbers (so complex conjugation plays no role, and everything will be linear in the strict sense). Let $f : X \rightarrow Y$ be a linear continuous mapping.

By the Riesz representation theorem, Hilbert spaces are isometric isomorphic to their own dual spaces. This leads to different notions of duality, which confuses me.

(i) The dual operator of $f^\ast$ is the operator $f^\ast : Y^\ast \rightarrow X^\ast$ defined by $y^\ast \mapsto ( x \mapsto y^\ast( f x ) )$

(ii) The dual operator is the adjoint, i.e. the unique operator $f^\ast : Y \rightarrow X$ such that $\forall x \in X, y \in Y : \langle fx,y \rangle_Y = \langle x,f^\ast y \rangle_X$

The transition between these two different notions seems to be full and faithfully functiorial. - Nevertheless, I would like to differ between these two notions of duality; just think in analysis of the Hilbert space $H^1_0$ and its dual $H^{-1}$. But I even don't know whether speaking about "different dualities" is not a crime.

So, I would to know whether there are different words for this, whether these are really different concepts and how to use these two appropiately.