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HighestWeights :: Example 7

Example 7 -- With the exceptional group G2

Consider the exceptional group of type $G_2$; we denote $V(a,b)$ the highest weight representation of $G_2$ with highest weight $a\omega_1 + b\omega_2$, where $\omega_1$ and $\omega_2$ are the fundamental weights for $G_2$ (see Fulton, Harris - Representation Theory, Ch, 22.3 for a construction of the representations appearing in this example). We work over the polynomial ring $Sym(V(1,0))$, which has a natural action of $G_2$. For lack of a better notation, we index the variables in $R$ by their weight (recall that variables in $R$ must be weight vectors). Consider the maximal ideal $m$ generated by the variables of $R$. This ideal is clearly stable under the action of $G_2$. Moreover the minimal free resolution of $R/I$ is the Koszul complex over the variables of $R$ (see Eisenbud - Commutative Algebra, Ch. 17).

i1 : R=QQ[x_(-2, 1),x_(-1, 0),x_(-1, 1),x_(0, 0),x_(1,-1),x_(1, 0),x_(2,-1)]

o1 = R

o1 : PolynomialRing
i2 : m=ideal vars R;

o2 : Ideal of R
i3 : K=koszul gens m; betti K

            0 1  2  3  4  5 6 7
o4 = total: 1 7 21 35 35 21 7 1
         0: 1 7 21 35 35 21 7 1

o4 : BettiTally

Now we define the list of weights of the variables and attach it to the ring.

i5 : W={{-2, 1},{-1, 0},{-1, 1},{0, 0},{1,-1},{1, 0},{2,-1}}

o5 = {{-2, 1}, {-1, 0}, {-1, 1}, {0, 0}, {1, -1}, {1, 0}, {2, -1}}

o5 : List
i6 : D=dynkinType{{"G",2}}; setWeights(R,D,W)

o7 = Tally{{1, 0} => 1}

o7 : Tally

We can ask to decompose the complex $K$ up to homological degree 3 using the option Range:

i8 : highestWeightsDecomposition(K,Range=>{0,3})

o8 = HashTable{0 => HashTable{{0} => Tally{{0, 0} => 1}}}
               1 => HashTable{{1} => Tally{{1, 0} => 1}}
               2 => HashTable{{2} => Tally{{0, 1} => 1}}
                                           {1, 0} => 1
               3 => HashTable{{3} => Tally{{0, 0} => 1}}
                                           {1, 0} => 1
                                           {2, 0} => 1

o8 : HashTable

Then the first half of $K$ is: $$R \leftarrow V(1,0) \otimes R(-1) \leftarrow (V(0,1)\oplus V(1,0)) \otimes R(-2) \leftarrow (V(0,0)\oplus V(1,0)\oplus V(2,0)) \otimes R(-3) \leftarrow \ldots $$ The second half can be reconstructed using the duality of the Koszul complex and the fact that all the representations in the first half are self dual.

While the quotient ring $R/m$ is isomorphic to ${\mathbb C}$, the trivial representation of $G_2$, it may be more interesting to decompose some graded components of $m$.

i9 : highestWeightsDecomposition(m,0,4)

o9 = HashTable{0 => Tally{}           }
               1 => Tally{{1, 0} => 1}
               2 => Tally{{0, 0} => 1}
                          {2, 0} => 1
               3 => Tally{{1, 0} => 1}
                          {3, 0} => 1
               4 => Tally{{0, 0} => 1}
                          {2, 0} => 1
                          {4, 0} => 1

o9 : HashTable

Since $m$ is generated by the variables, it contains all graded components of $R$ except for the one in degree zero. In particular, the graded components of $R$ and $m$ coincide in degree 1 and higher, which is illustrated up to degree 4 by the computation below.

i10 : highestWeightsDecomposition(R,0,4)

o10 = HashTable{0 => Tally{{0, 0} => 1}}
                1 => Tally{{1, 0} => 1}
                2 => Tally{{0, 0} => 1}
                           {2, 0} => 1
                3 => Tally{{1, 0} => 1}
                           {3, 0} => 1
                4 => Tally{{0, 0} => 1}
                           {2, 0} => 1
                           {4, 0} => 1

o10 : HashTable