In this preliminary chapter we recall some of the theoretic background of Lie rings and Lie p-rings. We refer to Chapter 5 in [Khu98] for some further details. Throughout we assume that p stands for a rational prime.
A Lie ring L is an additive abelian group with a multiplication that is alternating, bilinear and satisfies the Jacobi identity. We denote the product of two elements g and h of L with g h.
A subset I ⊆ L is an ideal in the Lie ring L if it is a subgroup of the additive group of L and it satisfies a l ∈ I for all a ∈ I and l ∈ L. As the multiplication in L is alternating, it follows that l a ∈ I for all l ∈ L and a ∈ I. Note that if I and J are ideals in L, then I + J = { a + b ∣ a ∈ I, b ∈ J} and I J = ⟨ a b ∣ a ∈ I, b ∈ J ⟩_+ are ideals in L.
A subset U ⊆ L is a subring of the Lie ring L if U is a Lie ring with respect to the addition and the multiplication of L. Every ideal in L is also a subring of L. As usual, for an ideal I in L the quotient L/I has the structure of a Lie ring, but this does not hold for subrings.
The lower central series of the Lie ring L is the series of ideals L = γ_1(L) ≥ γ_2(L) ≥ ... defined by γ_i(L) = γ_i-1(L) L. We say that L is nilpotent if there exists a natural number c with γ_c+1(L) = {0}. The smallest natural number with this property is the class of L.
The notion of nilpotence now allows us to state the central definition of this package. A Lie p-ring is a Lie ring that is nilpotent and has p^n elements for some natural number n.
Every finite dimensional Lie algebra over a field with p elements is an example of a Lie ring with p^n elements. Note that there exist non-nilpotent Lie algebras of this type: the Lie algebra consisting of all n × n matrices with trace 0 and n ≥ 3 is an example. Thus not every Lie ring with p^n elements is nilpotent. (In contrast to the group case, where every group with p^n elements is nilpotent!)
For a Lie p-ring L we define the series L = λ_1(L) ≥ λ_2(L) ≥ ... via λ_i+1(L) = λ_i(L) L + p λ_i(L). This series is the lower exponent-p central series of L. Its length is the p-class of L. If |L/λ_2(L)| = p^d, then d is the minimal generator number of L. Similar to the p-group case, one can observe that this is indeed the cardinality of a generating set of smallest possible size.
Each Lie p-ring L has a central series L = L_1 ≥ ... ≥ L_n ≥ {0} with quotients of order p. Choose l_i ∈ L_i ∖ L_i+1 for 1 ≤ i ≤ n. Then (l_1, ..., l_n) is a generating set of L satisfying that p l_i ∈ L_i+1 and l_i l_j ∈ L_i+1 for 1 ≤ j < i ≤ n. We call such a generating sequence a basis for L and we say that L has dimension n.
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