# minimalModel -- compute the minimal model

## Synopsis

• Usage:
M = minimalModel(d,L)
• Inputs:
• Outputs:

## Description

That $M$ is a minimal model of a Lie algebra $L$ up to degree $d$ means that there exists a differential Lie algebra homomorphism $f: M \ \to\ L$ such that $H(f)$ is an isomorphism up to degree $d$, $M$ is free as a Lie algebra, and the linear part of the differential on $M$ is zero. The homomorphism $f$ may be obtained using map(LieAlgebra) applied to $M$.

The generators of $M$ yield a basis for the cohomology of $L$, i.e., $Ext_{UL}(k,k)$, where $k$ is the coefficient field of $L$. This skewcommutative algebra may be obtained using extAlgebra. Multiplication of elements in $Ext_{UL}(k,k)$ is obtained using ExtElement ExtElement.

Observe that the homological weight in the cohomology algebra is one higher than the homological weight in the minimal model.

Since $R$ in the following example is a Koszul algebra it follows that the cohomology algebra of $L$ is equal to $R$. This means that the minimal model of $L$ has generators in each degree $(d,d-1)$.

 i1 : R=QQ[x] o1 = R o1 : PolynomialRing i2 : L=koszulDual R o2 = L o2 : LieAlgebra i3 : describe L o3 = generators => {ko } 0 Weights => {{1, 0}} Signs => {1} ideal => { - (1/2)(ko_0 ko_0)} ambient => LieAlgebra{...10...} diff => {} Field => QQ computedDegree => 0 i4 : E=extAlgebra(5,L) o4 = E o4 : ExtAlgebra i5 : dims(5,E) o5 = | 1 0 0 0 0 | | 0 1 0 0 0 | | 0 0 1 0 0 | | 0 0 0 1 0 | | 0 0 0 0 1 | 5 5 o5 : Matrix ZZ <--- ZZ i6 : describe minimalModel(5,L) o6 = generators => {fr , fr , fr , fr , fr } 0 1 2 3 4 Weights => {{1, 0}, {2, 1}, {3, 2}, {4, 3}, {5, 4}} Signs => {1, 1, 1, 1, 1} ideal => {} ambient => LieAlgebra{...10...} diff => {0, (fr_0 fr_0), (fr_0 fr_1), (fr_1 fr_1) + 4 (fr_0 fr_2), 2 (fr_1 fr_2) + (fr_0 fr_3)} Field => QQ computedDegree => 5 map => fr => ko_0 0 fr => 0 1 fr => 0 2 fr => 0 3 fr => 0 4 source => LieAlgebra{...10...} target => L

In the following example the enveloping algebra of $L1$ has global dimension $2$, which means that the computed minimal model is in fact the full minimal model of $L1$.

 i7 : L1=lieAlgebra{a,b,c}/{a b,a b c} o7 = L1 o7 : LieAlgebra i8 : M1= minimalModel(3,L1) o8 = M1 o8 : LieAlgebra i9 : describe M1 o9 = generators => {fr , fr , fr , fr , fr } 0 1 2 3 4 Weights => {{1, 0}, {1, 0}, {1, 0}, {2, 1}, {3, 1}} Signs => {0, 0, 0, 1, 1} ideal => {} ambient => LieAlgebra{...10...} diff => {0, 0, 0, (fr_1 fr_0), (fr_1 fr_2 fr_0)} Field => QQ computedDegree => 3 map => fr => a 0 fr => b 1 fr => c 2 fr => 0 3 fr => 0 4 source => M1 target => L1 i10 : H=lieHomology M1 o10 = H o10 : VectorSpace i11 : dims(6,L1)===dims(6,H) o11 = true