Every multiplicative linear functional on $\ell^{\infty}$ is the limit along an ultrafilter. It is well-known that for any ultrafilter $\mathscr{u}$ in $\mathbb{N}$, the map\begin{equation}a\mapsto \lim_{\mathscr{u}}a\end{equation} is a multiplicative linear functional, where $\lim_{\mathscr{u}}a$ is the limit of the sequence $a$ along $\mathscr{u}$.
I vaguely remember someone once told me that every multiplicative linear functional on $\ell^{\infty}$ is of this form. That is, given a multiplicative linear functional $h$ on $\ell^{\infty}$, there is an ultrafilter $\mathscr{u}$ such that \begin{equation}
h(a)=\lim_{\mathscr{u}}a
\end{equation} for all $a\in\ell^{\infty}$.
However, I cannot find a proof to this. I can show that if $h$ is the evaluation at $n$, then $h$ corresponds to the principal ultrafilter centered at $n$, but there are other kinds of multiplicative functionals (all these must vanish on any linear combinations of point masses though).
Can somebody give a hint on how to do this latter case?
Thanks!
 A: Let me give a slightly different approach to the second half of Martin Argerami's answer.  Suppose $\varphi:\ell^\infty\to\mathbb{C}$ is a multiplicative linear functional, and as in Martin Argerami's answer define $\mathcal{U}=\{A:\varphi(1_A)=1\}$ and prove that $\mathcal{U}$ is an ultrafilter.  Now we just need to prove that $$\varphi(c)=\lim\limits_\mathcal{U} c$$ for any $c\in\ell^\infty$.
To prove this, let $L=\lim\limits_\mathcal{U} c$ and fix $\epsilon>0$.  Since $L$ is the limit of $c$ along $\mathcal{U}$, the set $$A=\{n:|c(n)-L|<\epsilon\}$$ is in $\mathcal{U}$.  Let $d=c\cdot 1_A$.  Since $\varphi$ is multiplicative, $\varphi(d)=\varphi(c)\varphi(1_A)=\varphi(c)\cdot 1=\varphi(c)$.  But by definition of $A$, $\|d-L1_A\|\leq\epsilon$.  Thus $$|\varphi(c)-L|=|\varphi(c)-\varphi(L1_A)|=|\varphi(d)-\varphi(L1_A)|=|\varphi(d-L1_A)|\leq \|\varphi\|\epsilon.$$  Since $\epsilon$ was arbitrary, we conclude that $\varphi(c)=L$.
(Note that this argument, like Martin Argerami's, assumes $\varphi$ is bounded.  You don't actually need to assume this; you can prove it.  Indeed, $\|c\|$ can be described as the least $r\in [0,\infty)$ such that for all $\lambda\in\mathbb{C}$ such that $|\lambda|> r$, $\lambda-c$ has a multiplicative inverse.  Since $\varphi$ preserves multiplicative inverses, $\lambda-\varphi(c)\neq 0$ whenever $|\lambda|> \|c\|$, so $|\varphi(c)|\leq \|c\|$.)
A: There are several ways of doing this, but I'll go with the most "elementary". 
Let $\varphi$ be a nonzero multiplicative functional on $\ell^\infty(\mathbb{N})$. Since $\varphi(1)=\varphi(1^2)=\varphi(1)^2$, we get that $\varphi(1)=1$ (it cannot be zero, because then $\varphi=0$).
Now let $a\in\ell^\infty(\mathbb{N})$ such that $a(n)\in\{0,1\}$ for all $n$. Write $\alpha=\varphi(a)$. As $a(1-a)=0$, we have 
$$
0=\varphi(a(1-a))=\varphi(a)\varphi(1-a)=\alpha(1-\alpha).
$$
So either $\alpha=0$ or $\alpha=1$. 
Note that we can write $a=1_A$, $A\subset\mathbb{N}$, where $A=\{n: a(n)=1\}$. Now define
$$
\mathcal U=\{A:\ \varphi(1_A)=1\}.
$$
We can see that 


*

*$\mathbb{N}\in\mathcal U$ (since $\varphi(1)=1$)

*$A\in\mathcal U\ \iff\ A^c\not\in\mathcal U$ (because $1_A\,1_{A^c}=0$)

*If $A,B\in\mathcal U$, then $A\cap B\in\mathcal U$ (because $1_{A\cap B}=1_A\,1_B$)

*If $A\in\mathcal U$ and $A\subset B$, then $B\in\mathcal U$ (because $1_A=1_A\,1_B$)


In other words, $\mathcal U$ is an ultrafilter. 
Now let $c\in\ell^\infty(\mathbb{N})$ be positive, i.e. $0\leq c\leq 1$. Define sets
$$
A_j^{(n)}=\{m:\ \frac{j}{2^n}\leq c(m)<\frac{(j+1)}{2^n}\},\ \ j=0,1,\ldots,2^n.
$$
For fixed $n$, these sets are pairwise disjoint and $$\tag{1}\bigcup_jA_j^{(n)}=\mathbb{N}.$$
As $\mathcal U$ is an ultrafilter, for each $n$ there is exactly one $j(n)$ such that $A_{j(n)}^{(n)}\in\mathcal U$, and none of the others is (if $A\cup B=\mathbb N$, then either $A\in\mathcal U$ or $B=A^c\in\mathcal U$; by induction, this applies to arbitrary partitions of $\mathbb N$).
Define
$$
c_n=\sum_{j=0}^{2^n-1}\,\frac{j}{2^n}\,1_{A_j^{(n)}}.
$$
By definition, $\|c-c_n\|\leq 2^{-n}$, so $c_n\to c$ in norm. As $\varphi$ is norm-continuous, we have  $\varphi(c)=\lim_n\varphi(c_n)$. And
$$
\varphi(c_n)=\sum_{j=0}^{2^n-1}\,\frac{j}{2^n}\,\varphi(1_{A_j^{(n)}})=\frac{j(n)}{2^n},
$$
so
$$
\varphi(c)=\lim_n \ c(j(n))=\lim_{\mathcal U}\ c.
$$
Last step is to extend by linearity to all of $\ell^\infty(\mathbb{N})$.
