## Construction of A-Series Root Systems

Starting from our setup, we construct root systems corresponding to the Dynkin diagrams.

We start with the -dimensional space with orthonormal basis , and cut out the -dimensional subspace orthogonal to the vector . This consists of those vectors for which the coefficients sum to zero: . We let , consisting of the lattice vectors whose (integer) coefficients sum to zero. Finally, we define our root system to consist of those vectors such that .

From this construction it should be clear that consists of the vectors . The vectors are independent, and thus form a basis of the -dimensional space . This establishes that spans . In particular, if we can write

showing that forms a base for .

We calculate the Cartan integers for this base

For we get the value ; for or we get the value ; otherwise we get the value . This clearly gives us the Dynkin diagram .

Finally, the reflections with respect to the should generate the entire Weyl group. We must verify that these leave the lattice invariant to be sure that we have a root system. We calculate

That is, it swaps the coefficients of and , and thus sends the lattice back to itself, as we need.

We can also see from this effect that any combination of the serves to permute the coefficients of a given vector. That is, the Weyl group of the system is naturally isomorphic to the symmetric group .

[...] construction is similar to that of the series, but instead of starting with a hyperplane in -dimensional space, we just start with -dimensional [...]

Pingback by Construction of D-Series Root Systems « The Unapologetic Mathematician | March 3, 2010 |

[...] to the series, and exactly as in the series, we define for . This time, though, to get vectors whose [...]

Pingback by Construction of B- and C-Series Root Systems « The Unapologetic Mathematician | March 4, 2010 |

[...] root system is, as we can see by looking at it, closely related to the root system. And so we start again with the -dimensional subspace of consisting of vectors with coefficients [...]

Pingback by Construction of the G2 Root System « The Unapologetic Mathematician | March 8, 2010 |

There is a small type in the line “From this construction it should be clear that consists of the vectors..you missed a – sign there, and a pipe further down the line.

Comment by Sangeeta Bhatia | July 28, 2012 |