[Math] Classsifying 1- and 2- dimensional Algebras, up to Isomorphism

differential-geometrylie-algebras

I am trying to find all 1- or 2- dimensional Lie Algebras "a" up to isomorphism. This is what I have so far:

If a is 1-dimensional, then every vector (and therefore every tangent
vector field) is of the form $cX$. Then , by anti-symmetry, and bilinearity:

$$[X,cX]=c[X,X]= -c[X,X]==0$$

I think this forces a unique Lie algebra because Lie algebra isomorphisms preserve the bracket. I also know Reals $\mathbb{R}$ are the only 1-dimensional Lie group, so its Lie algebra ($\mathbb{R}$ also) is also 1-dimensional. How can I show that every other 1-dimensional algebra is isomorphic to this one? Do I use preservation of bracket?

For 2 dimensions, I am trying to use the fact that the dimension of the Lie algebra g of a group $G$ is the same as the dimension of the ambient group/manifold $G$. I know that all surfaces (i.e., groups of dimension 2) can be classified as products of spheres and Tori, and I think the only 2-dimensional Lie group is $S^1\times S^1$, but I am not sure every Lie algebra can be realized as the Lie algebra of a Lie group ( I think this is true in the finite-dimensional case, but I am not sure).

I know there is a result out there that I cannot yet prove that all 1- and
2-dimensional Lie algebras are isomorphic to Lie subalgebras of $GL(2,\mathbb{R})$ (using matrix multiplication, of course); would someone suggest how to show this last? Thanks.

Best Answer

I found myself working on this same problem (for homework), and I think I've written a fairly detailed solution. So I will post it here, in case it is helpful to anyone else.


Let $\mathfrak{g}$ be a 1-dimensional Lie algebra, and let $\{E_1\}$ be a basis for $\mathfrak{g}$. Then for any two vector fields $X,Y\in\mathfrak{g}$, we have $X=aE_1$ and $Y=bE_1$, for some $a,b\in\mathbb{R}$. Thus, $$[X,Y]=[aE_1,bE_1]=ab[E_1,E_1]=0$$ for all $X,Y\in\mathfrak{g}$. Therefore, the only 1-dimensional Lie algebra is the trivial one. The map $$\varphi:\mathfrak{g}\rightarrow\mathfrak{gl}(2,\mathbb{R})$$ $$\varphi:aE_1\mapsto \left(\begin{array}{ll} a&0\\ 0&0 \end{array}\right)$$ is a Lie algebra homomorphism, since $$\varphi([aE_1,bE_1])=\varphi(0)=\left(\begin{array}{ll} 0&0\\ 0&0 \end{array}\right)\mbox{, and}$$ $$[\varphi(aE_1),\varphi(bE_1)]=\left(\begin{array}{ll} a&0\\ 0&0 \end{array}\right)\left(\begin{array}{ll} b&0\\ 0&0 \end{array}\right)-\left(\begin{array}{ll} b&0\\ 0&0 \end{array}\right)\left(\begin{array}{ll} a&0\\ 0&0 \end{array}\right)$$ $$=\left(\begin{array}{ll} 0&0\\ 0&0 \end{array}\right).$$ Thus, $\mathfrak{g}$ is isomorphic to the (abelian) Lie subalgebra $$\varphi(\mathfrak{g})=\left\{\left(\begin{array}{ll} a&0\\ 0&0 \end{array}\right)\in\mathfrak{gl}(2,\mathbb{R}):a\in\mathbb{R}\right\}\subset\mathfrak{gl}(2,\mathbb{R}).$$

Now let $\mathfrak{h}$ be a 2-dimensional Lie algebra, and let $\{E_1,E_2\}$ be a basis for $\mathfrak{h}$. Then for any two vector fields $X,Y\in\mathfrak{h}$, we have $X=aE_1+bE_2$ and $Y=cE_1+dE_2$, for some $a,b,c,d\in\mathbb{R}$. Thus, $$\begin{array}{ll} [X,Y]&=[aE_1+bE_2,cE_1+dE_2]\\ &=a[E_1,cE_1+dE_2]+b[E_2,cE_1+dE_2]\\ &=ac[E_1,E_1]+ad[E_1,E_2]+bc[E_2,E_1]+bd[E_2,E_2]\\ &=(ad-bc)[E_1,E_2]. \end{array}$$

If $[E_1,E_2]=0$, then we have the trivial 2-dimensional Lie algebra. The map $$\varphi:\mathfrak{h}\rightarrow\mathfrak{gl}(2,\mathbb{R})$$ $$\varphi:aE_1+bE_2\mapsto \left(\begin{array}{ll} a&0\\ 0&b \end{array}\right)$$ is a Lie algebra homomorphism, since $$\varphi([aE_1+bE_2,cE_1+dE_2])=\varphi(0)=\left(\begin{array}{ll} 0&0\\ 0&0 \end{array}\right)\mbox{, and}$$

$$[\varphi(aE_1+bE_2),\varphi(cE_1+dE_2)]=\left(\begin{array}{ll} a&0\\ 0&b \end{array}\right)\left(\begin{array}{ll} c&0\\ 0&d \end{array}\right)-\left(\begin{array}{ll} c&0\\ 0&d \end{array}\right)\left(\begin{array}{ll} a&0\\ 0&b \end{array}\right)$$ $$=\left(\begin{array}{ll} 0&0\\ 0&0 \end{array}\right).$$Furthermore, this map is faithful (injective). Thus, $\mathfrak{h}$ is isomorphic to the (abelian) Lie subalgebra $$\varphi(\mathfrak{h})=\left\{\left(\begin{array}{ll} a&0\\ 0&b \end{array}\right)\in\mathfrak{gl}(2,\mathbb{R}):a,b\in\mathbb{R}\right\}\subset\mathfrak{gl}(2,\mathbb{R}).$$

If $[E_1,E_2]\neq0$, then set $E_3=[E_1,E_2]$. Then for all $X,Y\in\mathfrak{h}$ we have $[X,Y]=\lambda E_3$ for some $\lambda\in\mathbb{R}$. In particular, for any $E_4\in\mathfrak{g}$ such that $E_4$ and $E_3$ are linearly independent, we have $[E_4,E_3]=\lambda_0 E_3$. Replacing $E_4$ with $1/\lambda_0 E_4$, we now have a basis $\{E_4, E_3\}$ for $\mathfrak{g}$ such that $[E_4, E_3]=E_3$. The map $$\varphi:\mathfrak{h}\rightarrow\mathfrak{gl}(2,\mathbb{R})$$ $$\varphi:aE_4+bE_3\mapsto \left(\begin{array}{ll} a&b\\ 0&0 \end{array}\right)$$ is a Lie algebra homomorphism, since $$\varphi([aE_4+bE_3,cE_4+dE_3])=\varphi((ad-bc)E_3)=\left(\begin{array}{ll} 0&ad-bc\\ 0&0 \end{array}\right)\mbox{, and}$$

$$[\varphi(aE_4+bE_3),\varphi(cE_4+dE_3)]=\left(\begin{array}{ll} a&b\\ 0&0 \end{array}\right)\left(\begin{array}{ll} c&d\\ 0&0 \end{array}\right)-\left(\begin{array}{ll} c&d\\ 0&0 \end{array}\right)\left(\begin{array}{ll} a&b\\ 0&0 \end{array}\right)$$ $$=\left(\begin{array}{ll} 0&ad-bc\\ 0&0 \end{array}\right).$$Furthermore, this map is faithful (injective). Thus, $\mathfrak{h}$ is isomorphic to the (non-abelian) Lie subalgebra $$\varphi(\mathfrak{h})=\left\{\left(\begin{array}{ll} a&b\\ 0&0 \end{array}\right)\in\mathfrak{gl}(2,\mathbb{R}):a,b\in\mathbb{R}\right\}\subset\mathfrak{gl}(2,\mathbb{R}).$$

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