# What is the dual vector space of $\mathbb{C}\,$?

Given that the complex numbers $\mathbb{C}$ are considered to be canonically isomorphic to $\mathbb{R^2}$, I am wondering if the dual space of $\mathbb{C}$ is the same es that of $\mathbb{R}$, i.e. the vector space of linear functionals. I never heard about dual space of $\mathbb{C}$ until today, so I couldnt find much information in the litterature. Also:

• How is the dual of $\mathbb{C}$ defined, what is a dual basis of $\mathbb{C}$ ?
• What is the field $K$ in which the linear functionals get their values ($\mathbb{C}$ or $\mathbb{R}$) ?

Many thanks.

• The dual of any vector space $V$ over a field $K$ is the space of linear functionals $f:V\to K$. – A. Goodier Mar 5 '18 at 17:50
• For a finite dimensional vector space $\mathbb{F}^n$ (possibly barring some characteristic 2 issues - I'm a bit cloudy on it right now and fairly distraught, so I'm not sure about this), the dual space is $\mathbb{F}^n$. – Cameron Williams Mar 5 '18 at 17:51
• @CameronWilliams Why should characteristic $2$ be special here? It isn't. – egreg Mar 5 '18 at 18:17
• The Question has to tell Readers whether $\mathbb C$ is being considered a vector space over the field $\mathbb R$ or over the field $\mathbb C$. – hardmath Mar 5 '18 at 19:20
• @egreg I didn't think it was but I was having a particularly bad day today so I couldn't think well haha. – Cameron Williams Mar 5 '18 at 22:40

The answer to your questions depends upon your answer to this question: do you see $$\mathbb C$$ as a real vector space or as a complex vector space?

Since you mention $$\mathbb{R}^2$$, I'll assume that you see it as a real vector space. In that case:

• A basis of $$\mathbb{C}^*$$ is $$\{\alpha,\beta\}$$, with $$\alpha(z)=\operatorname{Re}z$$ and $$\beta(z)=\operatorname{Im}z$$.
• The functionals get their values in $$\mathbb R$$.

On the other hand, if you see $$\mathbb C$$ as a complex vector space, then you can take any map $$\alpha\colon\mathbb{C}\longrightarrow\mathbb{C}$$ of the type $$\alpha(z)=az$$ (as long as $$a\neq0$$) and $$\{\alpha\}$$ will be a basis of $$\mathbb{C}^*$$. For instance, you can take $$\alpha=\operatorname{Id}$$ (which corresponds to choosing $$a=1$$).

• Many thanks. So in that case is then obvious that $\alpha (1)=1, \alpha (i)=0$ and $\beta (1)=0, \beta (i)=1.$ How about if one considered $\mathbb{C}$ as a complex vector space ? – user249018 Mar 5 '18 at 17:59
• @user249018 Indeed. Of course, a shorter answer to the first question would have been $\{\operatorname{Re},\operatorname{Im}\}$. – José Carlos Santos Mar 5 '18 at 18:01
• Since {1} is the only basis vector of $\mathbb{C}$ considered as a complex vector space, one would expect a single dual basis vector, say $\theta$. I am wondering if $\theta (1)=1$ or $0$ ? – user249018 Mar 5 '18 at 18:10
• @user249018 Acutaly, for every $z\in\mathbb{C}\setminus\{0\}$, $\{z\}$ is a basis of $\mathbb C$, if we see it as a complex vector space. But, yes, then $\mathbb{C}^*$ will be $1$-dimensional. Please see the paragraph that I've added. – José Carlos Santos Mar 5 '18 at 18:15
• @user249018 No. What I wrote is what I meant: for each $z\in\mathbb{C}\setminus\{0\}$, $\{z\}$ is a basis of $\mathbb C$, if we see it as a complex vector space. Why do you ask? Do you know some non-zero complex number $z$ such that $\{z\}$ is not a basis of $\mathbb C$? – José Carlos Santos Mar 5 '18 at 18:26