# Root Data and Lie Types¶

To start let us talk a bit about Lie types. A Lie type is a product of Complex Lie Algebras of different types. By choosing a Lie type we specify a complex reductive Lie Algebra which is a product of simple and abelian factors. Then we choose a root datum for that Lie type. for example

atlas> set g=LieType:"A1T1"
Identifier g: LieType
atlas> g
Value: Lie type 'A1.T1'
atlas> set rd=simply_connected (g)
Identifier rd: RootDatum
atlas> rd
Value: simply connected root datum of Lie type 'A1.T1'
atlas>
atlas> whattype simply_connected ?
LieType->RootDatum
atlas>


This is what atlas understands as a root datum. The function simply_connected applied to a Lie type gives you a root datum. This is a set of simple roots and simple coroots.

Recall that a root datum consists of two lattices, $$X^*$$, $$X_*$$, with a perfect pairing: $$X^* \times X_* \rightarrow Z$$ (which in these coordinates corresponds to the dot product ) and subsets of simple roots and simple coroots respectively. In the above example we have:

atlas> simple_roots (rd)
Value:
| 2 |
| 0 |

atlas> simple_coroots (rd)
Value:
| 1 |
| 0 |

atlas>


In atlas, a root datum is a pair of m×n integral matrices (A,B) such that ^AB is the Cartan matrix for the Lie algebra $$\mathfrak g$$ of the given Lie Type, m is the rank of $$\mathfrak g$$, and n is the semisimple rank. In the above example, the rank is 2 and the semisimple rank is 1. And we have:

atlas> set A=simple_roots (rd)
Identifier A: mat (hiding previous one of type mat)
atlas> A
Value:
| 2 |
| 0 |

atlas> set B=simple_coroots (rd)
Identifier B: mat (hiding previous one of type mat)
atlas> B
Value:
| 1 |
| 0 |

atlas> ^A*B
Value:
| 2 |

atlas>


These are not necessarily the usual coordinates of this group. Another example is the Lie Type A1:

atlas> g:="A1"
Value: Lie type 'A1'
atlas> g
Value: Lie type 'A1'
atlas> set rd=simply_connected (g)
Identifier rd: RootDatum (hiding previous one of type RootDatum)
atlas> rd
Value: simply connected root datum of Lie type 'A1'
atlas> simple_roots (rd)
Value:
| 2 |

atlas> simple_coroots (rd)
Value:
| 1 |

atlas> simple_roots (rd)*simple_coroots (rd)
Value:
| 2 |


In this case we have the usual coordinates and the cartan matrix for A1 On the other hand, if we work with the adjoint group, we have the following situation:

atlas> rd:=adjoint (g)
Value: adjoint root datum of Lie type 'A1'
atlas> simple_roots (rd)
Value:
| 1 |

atlas> simple_coroots (rd)
Value:
| 2 |

atlas> simple_roots (rd)*simple_coroots (rd)
Value:
| 2 |


Here the coordinates of the simple roots and coroots are interchanged. In these coordinates, for the simply connected case, this means that $$X^* ={\mathbb Z}$$ and the simple root is alpha=, so alpha/2= is in $$X^*$$.

On the other hand, for the adjoint group, we have that $$X^*$$ is still $$\mathbb Z$$, the simple root is alpha= and alpha/2 =/2 is NOT in $$X^*$$.

In general $$X^*$$ and $$X_*$$ by definition are $${\mathbb Z}^n$$. However, whether the roots or coroots lie in them, has to do with isogenies of the group.

Another useful .at file that lets you put in Lie types without the quotation marks is the file “lietypes.at”. It is also included in the all.at file. This file just defines the Lie types as strings

atlas> A1
Value: "A1"
atlas>whattype A1
type: string


And now we can type:

atlas> set g=LieType :A1
Identifier g: LieType (hiding previous one of type string)
atlas> g
Value: Lie type 'A1'
atlas> whattype A1
type: string


Or we can say:

atlas> set rd=simply_connected (A1)
Identifier rd: RootDatum (hiding previous one of type RootDatum)
atlas> rd
Value: simply connected root datum of Lie type 'A1'
atlas>