<html><head><title>[LiePRing] 2 Lie p-rings</title></head>
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<h1>2 Lie p-rings</h1><p>
<p>
In this preliminary chapter we recall some of theoretic background
of Lie rings and Lie <i>p</i>-rings. We refer to Chapter 5 in <a href="biblio.htm#Khu98"><cite>Khu98</cite></a>
for some further details. Throughout we assume that <i>p</i> stands for
a rational prime.
<p>
<p>
A Lie ring <i>L</i> is an additive abelian group with a multiplication that
is alternating, bilinear and satisfies the Jacobi identity. We denote
the product of two elements <i>g</i> and <i>h</i> of <i>L</i> with <i>g</i> <i>h</i>.
<p>
<p>
A subset <i>I</i> ⊆ <i>L</i> is an <em>ideal</em>in the Lie ring <i>L</i> if it
is a subgroup of the additive group of <i>L</i> and it satisfies <i>a</i> <i>l</i> ∈ <i>I</i> for all <i>a</i> ∈ <i>I</i> and <i>l</i> ∈ <i>L</i>. As the multiplication in <i>L</i> is
alternating, it follows that <i>l</i> <i>a</i> ∈ <i>I</i> for all <i>l</i> ∈ <i>L</i> and <i>a</i> ∈ <i>I</i>. Note that if <i>I</i> and <i>J</i> are ideals in <i>L</i>, then <i>I</i> + <i>J</i> = { <i>a</i> + <i>b</i> | <i>a</i> ∈ <i>I</i>, <i>b</i> ∈ <i>J</i>} and <i>I</i> <i>J</i> = 〈<i>a</i> <i>b</i> | <i>a</i> ∈ <i>I</i>, <i>b</i> ∈ <i>J</i> 〉<sub>+</sub> are ideals in <i>L</i>.
<p>
<p>
A subset <i>U</i> ⊆ <i>L</i> is a <em>subring</em>of the Lie ring <i>L</i> if <i>U</i>
is a Lie ring with respect to the addition and the multiplication of <i>L</i>.
Every ideal in <i>L</i> is also a subring of <i>L</i>. As usual, for an ideal <i>I</i> in
<i>L</i> the quotient <i>L</i>/<i>I</i> has the structure of a Lie ring, but this does not
hold for subrings.
<p>
<p>
The <em>lower central series</em>of the Lie ring <i>L</i> is the series of ideals
<i>L</i> = γ<sub>1</sub>(<i>L</i>) ≥ γ<sub>2</sub>(<i>L</i>) ≥ … defined by γ<sub><i>i</i></sub>(<i>L</i>) = γ<sub><i>i</i>−1</sub>(<i>L</i>) <i>L</i>. We say that <i>L</i> is <em>nilpotent</em>if there exists a
natural number <i>c</i> with γ<sub><i>c</i>+1</sub>(<i>L</i>) = {0}. The smallest natural number
with this property is the <em>class</em>of <i>L</i>.
<p>
<p>
The notion of nilpotence now allows to state the central definition of
this package. A <hr>Lie p-ring is a Lie ring that is nilpotent and has
<i>p</i><sup><i>n</i></sup> elements for some natural number <i>n</i>.
<p>
<p>
Every finite dimensional Lie algebra over a field with <i>p</i> elements
is an example for a Lie ring with <i>p</i><sup><i>n</i></sup> elements. Note that there exist
non-nilpotent Lie algebras of this type: the Lie algebra consisting of
all <i>n</i> ×<i>n</i> matrices with trace 0 and <i>n</i> ≥ 3 is an example.
Thus not every Lie ring with <i>p</i><sup><i>n</i></sup> elements is nilpotent. (In contrast
to the group case, where every group with <i>p</i><sup><i>n</i></sup> elements is nilpotent!)
<p>
<p>
For a Lie <i>p</i>-ring <i>L</i> we define the series <i>L</i> = λ<sub>1</sub>(<i>L</i>) ≥ λ<sub>2</sub>(<i>L</i>) ≥ …
via λ<sub><i>i</i>+1</sub>(<i>L</i>) = λ<sub><i>i</i></sub>(<i>L</i>) <i>L</i> + <i>p</i> λ<sub><i>i</i></sub>(<i>L</i>). This
series is the <em>lower exponent-<i>p</i> central series</em>of <i>L</i>. Its length
is the <em><i>p</i>-class</em>of <i>L</i>. If |<i>L</i>/λ<sub>2</sub>(<i>L</i>)| = <i>p</i><sup><i>d</i></sup>, then <i>d</i> is
the <em>minimal generator number</em>of <i>L</i>. Similar to the <i>p</i>-group case,
one can observe that this is indeed the cardinality of a generating set
of smallest possible size.
<p>
<p>
Each Lie <i>p</i>-ring <i>L</i> has a central series <i>L</i> = <i>L</i><sub>1</sub> ≥ … ≥ <i>L</i><sub><i>n</i></sub> ≥ {0} with quotients of order <i>p</i>. Choose <i>l</i><sub><i>i</i></sub> ∈ <i>L</i><sub><i>i</i></sub> \<i>L</i><sub><i>i</i>+1</sub> for 1 ≤ <i>i</i> ≤ <i>n</i>. Then (<i>l</i><sub>1</sub>, …, <i>l</i><sub><i>n</i></sub>) is a generating
set of <i>L</i> satisfying that <i>p</i> <i>l</i><sub><i>i</i></sub> ∈ <i>L</i><sub><i>i</i>+1</sub> and <i>l</i><sub><i>i</i></sub> <i>l</i><sub><i>j</i></sub> ∈ <i>L</i><sub><i>i</i>+1</sub> for 1 ≤ <i>j</i> < <i>i</i> ≤ <i>n</i>. We call such a generating sequence
a <em>basis</em>for <i>L</i> and we say that <i>L</i> has <em>dimension</em><i>n</i>.
<p>
<p>
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<address>LiePRing manual<br>June 2024
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