RootString - Maple Help

LieAlgebras[RootString] - find the sequence of roots through a given root of a semi-simple Lie algebra

Calling Sequences

RootString(${\mathbf{α}}$, ${\mathbf{β}}$, ${\mathbf{Δ}}$, option)

Parameters

$\mathrm{α}$       - a vector, defining a root vector for a semi-simple Lie algebra

$\mathrm{β}$       - a vector, defining a root vector for a semi-simple Lie algebra

$\mathrm{Δ}$       - a list of vectors, defining a list of root vectors for a semi-simple Lie algebra and containing $\mathrm{α}$ and $\mathrm{β}$

option       - the keyword argument output = "stringlengths"

Description

 • Let $\mathrm{𝔤}$ be a semi-simple Lie algebra, $\mathrm{𝔥}$ a Cartan subalgebra, and the associated set of roots. If then the $\mathrm{α}$-string through $\mathrm{β}$ is the maximal sequence of roots of the form

where $p,q$ are non-negative integers.

 • The calling sequence RootString(${\mathbf{α}}$, ${\mathbf{β}}$, ${\mathbf{Δ}}$) returns the $\mathrm{α}$-string of roots through $\mathrm{β}$. The calling sequence RootString(${\mathbf{α}}$, ${\mathbf{β}}$, ${\mathbf{Δ}}$, output = "stringlengths") returns the list of non-negative integers .

Examples

 > $\mathrm{with}\left(\mathrm{DifferentialGeometry}\right):$$\mathrm{with}\left(\mathrm{LieAlgebras}\right):$

Example 1.

We initialize the split real form of the exceptional Lie algebra ${g}_{2}$ and retrieve the root space decomposition and the list of all roots. We then calculate some root strings. The structure equations for ${g}_{2}$ are obtained using SimpleLieAlgebraData.

 > $\mathrm{LD}≔\mathrm{SimpleLieAlgebraData}\left("g\left(2,Split\right)",\mathrm{g2}\right)$
 ${\mathrm{LD}}{:=}\left[\left[{\mathrm{e1}}{,}{\mathrm{e3}}\right]{=}{2}{}{\mathrm{e3}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e4}}\right]{=}{-}{3}{}{\mathrm{e4}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e5}}\right]{=}{-}{\mathrm{e5}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e6}}\right]{=}{\mathrm{e6}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e7}}\right]{=}{3}{}{\mathrm{e7}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e9}}\right]{=}{-}{2}{}{\mathrm{e9}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e10}}\right]{=}{3}{}{\mathrm{e10}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e11}}\right]{=}{\mathrm{e11}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e12}}\right]{=}{-}{\mathrm{e12}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e13}}\right]{=}{-}{3}{}{\mathrm{e13}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e3}}\right]{=}{-}{\mathrm{e3}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e4}}\right]{=}{2}{}{\mathrm{e4}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e5}}\right]{=}{\mathrm{e5}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e7}}\right]{=}{-}{\mathrm{e7}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e8}}\right]{=}{\mathrm{e8}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e9}}\right]{=}{\mathrm{e9}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e10}}\right]{=}{-}{2}{}{\mathrm{e10}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e11}}\right]{=}{-}{\mathrm{e11}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e13}}\right]{=}{\mathrm{e13}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e14}}\right]{=}{-}{\mathrm{e14}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e4}}\right]{=}{\mathrm{e5}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e5}}\right]{=}{2}{}{\mathrm{e6}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e6}}\right]{=}{-}{3}{}{\mathrm{e7}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e9}}\right]{=}{-}{\mathrm{e1}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e11}}\right]{=}{-}{3}{}{\mathrm{e10}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e12}}\right]{=}{-}{2}{}{\mathrm{e11}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e13}}\right]{=}{\mathrm{e12}}{,}\left[{\mathrm{e4}}{,}{\mathrm{e7}}\right]{=}{-}{\mathrm{e8}}{,}\left[{\mathrm{e4}}{,}{\mathrm{e10}}\right]{=}{-}{\mathrm{e2}}{,}\left[{\mathrm{e4}}{,}{\mathrm{e11}}\right]{=}{\mathrm{e9}}{,}\left[{\mathrm{e4}}{,}{\mathrm{e14}}\right]{=}{\mathrm{e13}}{,}\left[{\mathrm{e5}}{,}{\mathrm{e6}}\right]{=}{-}{3}{}{\mathrm{e8}}{,}\left[{\mathrm{e5}}{,}{\mathrm{e9}}\right]{=}{3}{}{\mathrm{e4}}{,}\left[{\mathrm{e5}}{,}{\mathrm{e10}}\right]{=}{-}{\mathrm{e3}}{,}\left[{\mathrm{e5}}{,}{\mathrm{e11}}\right]{=}{-}{\mathrm{e1}}{-}{3}{}{\mathrm{e2}}{,}\left[{\mathrm{e5}}{,}{\mathrm{e12}}\right]{=}{2}{}{\mathrm{e9}}{,}\left[{\mathrm{e5}}{,}{\mathrm{e14}}\right]{=}{\mathrm{e12}}{,}\left[{\mathrm{e6}}{,}{\mathrm{e9}}\right]{=}{2}{}{\mathrm{e5}}{,}\left[{\mathrm{e6}}{,}{\mathrm{e11}}\right]{=}{-}{2}{}{\mathrm{e3}}{,}\left[{\mathrm{e6}}{,}{\mathrm{e12}}\right]{=}{-}{2}{}{\mathrm{e1}}{-}{3}{}{\mathrm{e2}}{,}\left[{\mathrm{e6}}{,}{\mathrm{e13}}\right]{=}{-}{\mathrm{e9}}{,}\left[{\mathrm{e6}}{,}{\mathrm{e14}}\right]{=}{-}{\mathrm{e11}}{,}\left[{\mathrm{e7}}{,}{\mathrm{e9}}\right]{=}{-}{\mathrm{e6}}{,}\left[{\mathrm{e7}}{,}{\mathrm{e12}}\right]{=}{\mathrm{e3}}{,}\left[{\mathrm{e7}}{,}{\mathrm{e13}}\right]{=}{-}{\mathrm{e1}}{-}{\mathrm{e2}}{,}\left[{\mathrm{e7}}{,}{\mathrm{e14}}\right]{=}{-}{\mathrm{e10}}{,}\left[{\mathrm{e8}}{,}{\mathrm{e10}}\right]{=}{-}{\mathrm{e7}}{,}\left[{\mathrm{e8}}{,}{\mathrm{e11}}\right]{=}{-}{\mathrm{e6}}{,}\left[{\mathrm{e8}}{,}{\mathrm{e12}}\right]{=}{\mathrm{e5}}{,}\left[{\mathrm{e8}}{,}{\mathrm{e13}}\right]{=}{\mathrm{e4}}{,}\left[{\mathrm{e8}}{,}{\mathrm{e14}}\right]{=}{-}{\mathrm{e1}}{-}{2}{}{\mathrm{e2}}{,}\left[{\mathrm{e9}}{,}{\mathrm{e10}}\right]{=}{\mathrm{e11}}{,}\left[{\mathrm{e9}}{,}{\mathrm{e11}}\right]{=}{2}{}{\mathrm{e12}}{,}\left[{\mathrm{e9}}{,}{\mathrm{e12}}\right]{=}{-}{3}{}{\mathrm{e13}}{,}\left[{\mathrm{e10}}{,}{\mathrm{e13}}\right]{=}{-}{\mathrm{e14}}{,}\left[{\mathrm{e11}}{,}{\mathrm{e12}}\right]{=}{-}{3}{}{\mathrm{e14}}\right]$ (2.1)

Initialize the Lie algebra with DGsetup.

 > $\mathrm{DGsetup}\left(\mathrm{LD}\right)$
 ${\mathrm{Lie algebra: g2}}$ (2.2)

The root space decomposition is retrieved, without calculation, using SimpleLieAlgebraProperties

 > $P≔\mathrm{SimpleLieAlgebraProperties}\left(\mathrm{g2}\right):$

Here is the root space decomposition and the list of all positive roots.

 > $\mathrm{RSD}≔\mathrm{eval}\left(P\left["RootSpaceDecomposition"\right]\right)$
 ${\mathrm{RSD}}{:=}{\mathrm{table}}\left(\left[\left[{-}{1}{,}{1}\right]{=}{\mathrm{e5}}{,}\left[{-}{3}{,}{2}\right]{=}{\mathrm{e4}}{,}\left[{0}{,}{1}\right]{=}{\mathrm{e8}}{,}\left[{3}{,}{-}{1}\right]{=}{\mathrm{e7}}{,}\left[{2}{,}{-}{1}\right]{=}{\mathrm{e3}}{,}\left[{1}{,}{0}\right]{=}{\mathrm{e6}}{,}\left[{0}{,}{-}{1}\right]{=}{\mathrm{e14}}{,}\left[{-}{2}{,}{1}\right]{=}{\mathrm{e9}}{,}\left[{-}{3}{,}{1}\right]{=}{\mathrm{e13}}{,}\left[{3}{,}{-}{2}\right]{=}{\mathrm{e10}}{,}\left[{-}{1}{,}{0}\right]{=}{\mathrm{e12}}{,}\left[{1}{,}{-}{1}\right]{=}{\mathrm{e11}}\right]\right)$ (2.3)
 g2 > $\mathrm{\Delta }≔\mathrm{LieAlgebraRoots}\left(\mathrm{RSD}\right)$

Define two roots and $\mathrm{β}$.

 g2 > $\mathrm{\alpha }≔⟨2,-1⟩$
 g2 > $\mathrm{\beta }≔⟨-3,2⟩$

Calculate the $\mathrm{α}$-string through $\mathrm{β}.$

 g2 > $\mathrm{RootString}\left(\mathrm{\alpha },\mathrm{\beta },\mathrm{\Delta }\right)$

With the optional keyword argument output = "stringlengths", we obtain the lengths of the $\mathrm{α}$-string through in the negative and positive directions.

 g2 > $\mathrm{RootString}\left(\mathrm{\alpha },\mathrm{\beta },\mathrm{\Delta },\mathrm{output}="stringlengths"\right)$
 $\left[{0}{,}{3}\right]$ (2.4)

Thus the $\mathrm{α}$-string through $\mathrm{β}$ in  is given explicitly by.

 g2 > $\left[\mathrm{\beta },\mathrm{\beta }+\mathrm{\alpha },\mathrm{\beta }+2\mathrm{\alpha },\mathrm{\beta }+3\mathrm{\alpha }\right]$



Example 2.

Here is another example of a root string for the exceptional Lie algebra ${g}_{2}$.

 g2 > $\mathrm{\alpha }≔⟨2,-1⟩$
 g2 > $\mathrm{\beta }≔⟨1,0⟩$

The root $\mathrm{α}$-string through $\mathrm{β}$ is now

 g2 > $\mathrm{RootString}\left(\mathrm{\alpha },\mathrm{\beta },\mathrm{\Delta }\right)$

and the string lengths are

 g2 > $\mathrm{RootString}\left(\mathrm{\alpha },\mathrm{\beta },\mathrm{\Delta },\mathrm{output}="stringlengths"\right)$
 $\left[{2}{,}{1}\right]$ (2.5)



Thus, the root string  is explicitly given by

 g2 > $\left[\mathrm{\beta }-2\mathrm{\alpha },\mathrm{\beta }-\mathrm{\alpha },\mathrm{\beta },\mathrm{\beta }+\mathrm{\alpha }\right]$

Example 3.

Root strings can also be calculated for abstract roots systems, that is, a set of vectors satisfying the standard axioms of a root system and not explicitly defined from the root space decomposition of a semi-simple Lie algebra. The positive roots of an abstract root systems can be calculated with the PositiveRoots command.

 g2 > $\mathrm{AbstractRoots}≔\mathrm{PositiveRoots}\left("B",3\right)$
 g2 > $\mathrm{\Delta }≔\left[\mathrm{seq}\left(-v,v=\mathrm{AbstractRoots}\right),\mathrm{seq}\left(v,v=\mathrm{AbstractRoots}\right)\right]$

Here are are 2 roots and their root string.

 g2 > $\mathrm{\alpha }≔\mathrm{\Delta }\left[12\right]$
 g2 > $\mathrm{\beta }≔\mathrm{\Delta }\left[11\right]$
 g2 > $\mathrm{RootString}\left(\mathrm{\alpha },\mathrm{\beta },\mathrm{\Delta }\right)$