# Injective linear transformation from a vector space to the dual space of its dual space

I am currently trying to understand some concepts from some Linear Algebra. I seem to be having quite some difficulty understanding dual spaces and their dual spaces. I found this problem and was wondering how to get started on it.

Let $V$ be a vector space over the field $F$. Let $V^{*}$ be the dual space of $V$ and let $V^{**}$ be the dual space of $V^{*}$. Show that there is an injective linear transformation $\phi : V \rightarrow V^{**}$.

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Can you show that there is an injective linear transformation between a space $V$ and its dual $V^*$? If you can, then it's just one more line to answer your question. – Raskolnikov Jul 24 '12 at 6:53
Define the evaluation maps $\phi_v: V^*\to F:f\mapsto f(v)$ for each $v\in V$. Note that $\phi_v\in (V^*)^*$. Then consider $V\to V^{**}:v\mapsto \phi_v$. – anon Jul 24 '12 at 6:54
@ Raskolinikov..Is there any way you can help me get started with showing that there is an injective linear transformation between a space $V$ and its dual $V^{*}$, I am having a hard time understanding these concepts. Thanks. – Melky Jul 24 '12 at 8:13
Just try to construct some nontrivial linear map $V\to V^*$...there is a very obvious one! Then check injectivity. – wildildildlife Jul 24 '12 at 10:25

The canonical answer is the one given by anon above. The point is that when you write $f(v)$, for $v\in V$, $f\in V^*$, you can see it as "$f$ acting on $v$", or you can also see it as "$v$ acting on $f$"; this second point of view defines the injection you are looking for. The physicists write $f(v)$ as $\langle f,v\rangle$ to emphasize this duality, and that's where the word "dual" comes from.