Advances in the Theory of Varieties of Semigroups

Preface
Contents
1 Historical Overview and Main Results
	1.1 Important Varietal Properties
		1.1.1 Finite Basis Problem
		1.1.2 Hereditary Finite Basis Property
		1.1.3 Cross Varieties
	1.2 Varieties Generated by Completely 0-Simple Semigroups
		1.2.1 Rees–Suschkewitsch Varieties
		1.2.2 The Varieties A2 and B2
		1.2.3 Aperiodic Rees–Suschkewitsch Varieties
		1.2.4 Rees–Suschkewitsch Varieties Containing Nontrivial Groups
		1.2.5 Aperiodic Rees–Suschkewitsch Monoids
	1.3 Hereditary Finite Basis Property
		1.3.1 Hereditarily Finitely Based Identities
		1.3.2 Pseudo-Simple Hereditarily Finitely Based Identities
		1.3.3 Minimal Non-Finitely Based Semigroups
	1.4 Non-Finite Basis Property
		1.4.1 Finite Basis Problem for Finite Semigroups
		1.4.2 Establishing the Non-Finite Basis Property
			1.4.2.1 Critical Rees Matrix Semigroups
			1.4.2.2 Inherently Non-Finitely Based Finite Semigroups
			1.4.2.3 Syntactic Method
			1.4.2.4 A Comparison of the Three Methods
		1.4.3 Irredundant Identity Bases
	1.5 Varieties of Involution Semigroups
		1.5.1 Equational Properties of Involution Semigroups
		1.5.2 Lattice of Varieties of Involution Semigroups
		1.5.3 Relationship Between an Involution Semigroup and Its Semigroup Reduct
			1.5.3.1 Non-Twisted Involution Semigroups
			1.5.3.2 Inherent Non-Finite Basis Property
			1.5.3.3 Sufficient Conditions for the Non-Finite Basis Property
	1.6 Varieties of Monoids
		1.6.1 Rees Quotients of Free Monoids
		1.6.2 Limit Varieties and Hereditarily Finitely Based Varieties
		1.6.3 Cross Varieties and Inherently Non-Finitely GeneratedVarieties
		1.6.4 Further Examples Involving Rees Quotients of Free Monoids
2 Preliminaries
	2.1 Identities and Deducibility
	2.2 Varieties and Identity Bases
	2.3 Connected Words and Identities
	2.4 Rees Quotients of Free Monoids
	2.5 Involution Semigroups
		2.5.1 Terms, Words, and Plain Words
		2.5.2 Identities and Deducibility
Part I Semigroups
	3 Aperiodic Rees–Suschkewitsch Varieties
		3.1 Background Information on L(A2)
			3.1.1 Identity Bases for Some Subvarieties of A2
			3.1.2 Identities Defining Varieties in [A0,A2]
			3.1.3 A Decomposition of L(A2-)
		3.2 Finite Basis Property for Subvarieties of A2
			3.2.1 Varieties in I2=[A0,B2-]
			3.2.2 Varieties in I1=[A0vB2,A2-]
			3.2.3 Varieties in I3=[B2,A0-] and I4=[B0,A0-B2-]
			3.2.4 Varieties in I5=L(B0-)
		3.3 The Lattice L(A2)
			3.3.1 The Interval I5=L(B0-)
			3.3.2 The Varieties Dl, E, F, i
				3.3.2.1 The Varieties Dl
				3.3.2.2 The Varieties E and F
				3.3.2.3 The Varieties i
			3.3.3 The Intervals I1=[A0vB2,A2-] and I2=[A0,B2-]
			3.3.4 The Intervals I3=[B2,A0-] and I4=[B0,A0-B2-]
			3.3.5 The Interval [B0,A2]
		3.4 Subvarieties of A2 That Are Cross, Finitely Generated, or Small
			3.4.1 Cross Subvarieties and Small Subvarieties of A2
			3.4.2 Finitely Generated Subvarieties of A2
		3.5 Summary
	4 Pseudo-Simple Hereditarily Finitely Based Identities
		4.1 Non-Homotypical Identities
		4.2 Homotypical Identities
		4.3 Summary
	5 Sufficient Conditions for the Non-Finite Basis Property
		5.1 Identities Satisfied by L3
		5.2 Proof of Theorem 5.1
		5.3 Specialized Versions of Theorem 5.1
		5.4 Summary
	6 Semigroups Without Irredundant Identity Bases
		6.1 Sufficient Condition for the Nonexistence of Irredundant IdentityBases
		6.2 Identities Satisfied by L3,n
		6.3 Sandwich Identities
		6.4 Restrictions on Sandwich Identities
			6.4.1 Level of Sandwiches Forming Sandwich Identities
			6.4.2 Refined Sandwich Identities
		6.5 An Explicit Identity Basis for L3,n
		6.6 Nonexistence of Irredundant Identity Bases for L3,n
		6.7 Summary
Part II Involution Semigroups
	7 Involution Semigroups with Infinite Irredundant Identity Bases
		7.1 Identities Satisfied by 
		7.2 Connected Identities and -Sandwich Identities
			7.2.1 Connected Identities
			7.2.2 *-Sandwich Identities
		7.3 Restrictions on *-Sandwich Identities
			7.3.1 Type of *-Sandwiches Forming *-Sandwich Identities
			7.3.2 Refined *-Sandwich Identities
		7.4 An Explicit Identity Basis for  with R>1
			7.4.1 An Identity Basis from (7.1)
			7.4.2 A Simpler Identity Basis
		7.5 An Infinite Irredundant Identity Basis for  with R>1
			7.5.1 The Identities (7.4j)
			7.5.2 Proof of Theorem 7.21
		7.6 Smaller Examples
		7.7 Summary
	8 Finitely Based Involution Semigroups with Non-Finitely Based Reducts
		8.1 Identities and *-Sandwich Identities Satisfied by 
		8.2 Restrictions on *-Sandwich Identities
		8.3 An Explicit Identity Basis for 
		8.4 A Finite Identity Basis for 
		8.5 Summary
	9 Counterintuitive Examples of Involution Semigroups
		9.1 Involution Semigroups with Different Types of Identity Bases
			9.1.1 Involution Semigroups with an Irredundant Identity Basis
			9.1.2 Involution Semigroups Without Irredundant Identity Bases
		9.2 Two Incomparable Chains of Varieties of Involution Semigroups
			9.2.1 The Involution Semigroups 
			9.2.2 The Involution Semigroups 
			9.2.3 Proof of Theorem 9.10
		9.3 Summary
	10 Equational Theories of Twisted Involution Semigroups
		10.1 Organized Identity Bases
		10.2 Proof of Theorem 10.1
		10.3 Summary
Part III Monoids
	11 Hereditarily Finitely Based Varieties of Monoids
		11.1 Identities Satisfied by Noncommutative Subvarieties of O
			11.1.1 Canonical Form
			11.1.2 Fundamental Identities and Well-Balanced Identities
			11.1.3 Proof of Proposition 11.2
		11.2 Finite Basis Property of Subvarieties of O
		11.3 Distinguished Varieties
		11.4 Summary
	12 Varieties of Aperiodic Monoids with Central Idempotents
		12.1 Rigid Identities
			12.1.1 Definition and Basic Properties
			12.1.2 Straubing Identities
			12.1.3 Limiting Identities
		12.2 The Variety K
			12.2.1 Almost Cross Property
			12.2.2 Subvarieties of K
		12.3 Cross Subvarieties of Azen
		12.4 Varieties Inherently Non-Finitely Generated Within Azen
		12.5 A Non-Finitely Generated Subvariety of RQx2y2
		12.6 Summary
	13 Certain Cross Varieties of Aperiodic Monoids with Commuting Idempotents
		13.1 The Variety Q1
		13.2 Varieties that Contain Q1
		13.3 Varieties that Exclude K
		13.4 Proof of Theorem 13.1
		13.5 Summary
	14 Counterintuitive Examples of Monoids
		14.1 The Direct Product of RQxyx with Noncommutative Groups of Finite Exponent
			14.1.1 Identities Satisfied by RQxyx and by Noncommutative Groups
			14.1.2 Proof of Theorem 14.1
		14.2 Finitely Based Monoids from Non-Finitely Based Semigroups
		14.3 Summary
References
List of Symbols
	List of Symbols
		General Symbols
		Aspects of a General Word w
		Words
		Identities
		Finite Algebras
		Varieties of Semigroups
		Varieties or Classes of Monoids
Index