Differences

—		shells [2010/07/29 15:46] (current)
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		+	=====Shells=====
		+
		+	====Definition====
		+	A \emph{shell} is a structure $\mathbf{S}=\langle S,+,0,\cdot,1
		+	\rangle $ of type $\langle 2,0,2,0\rangle $ such that
		+
		+
		+	$0$ is an identity for $+$:  $0+x=x$, $x+0=x$
		+
		+
		+	$1$ is an identity for $\cdot$:  $1\cdot x=x$, $x\cdot 1=x$
		+
		+
		+	$0$ is a zero for $\cdot$:  $0\cdot x=0$, $x\cdot 0=0$
		+
		+	==Morphisms==
		+	Let $\mathbf{S}$ and $\mathbf{T}$ be shells. A morphism from $\mathbf{S}$
		+	to $\mathbf{T}$ is a function $h:S\rightarrow T$ that is a homomorphism:
		+
		+	$h(x+y)=h(x)+h(y)$, $h(x\cdot y)=h(x)\cdot h(y)$, $h(0)=0$, $h(1)=1$
		+
		+	====Examples====
		+	Example 1:
		+
		+	====Basic results====
		+
		+	====Properties====
		+	^[[Classtype]]  |variety |
		+	^[[Equational theory]]  |decidable |
		+	^[[Quasiequational theory]]  | |
		+	^[[First-order theory]]  |undecidable |
		+	^[[Locally finite]]  |no |
		+	^[[Residual size]]  |unbounded |
		+	^[[Congruence distributive]]  |no |
		+	^[[Congruence modular]]  |no |
		+	^[[Congruence n-permutable]]  |no |
		+	^[[Congruence regular]]  |no |
		+	^[[Congruence uniform]]  |no |
		+	^[[Congruence extension property]]  |no |
		+	^[[Definable principal congruences]]  |no |
		+	^[[Equationally def. pr. cong.]]  |no |
		+	^[[Amalgamation property]]  |yes |
		+	^[[Strong amalgamation property]]  |yes |
		+	^[[Epimorphisms are surjective]]  | |
		+	====Finite members====
		+
		+	$\begin{array}{lr}
		+	f(1)= &1\\
		+	f(2)= &\\
		+	f(3)= &\\
		+	f(4)= &\\
		+	f(5)= &\\
		+	f(6)= &\\
		+	\end{array}$
		+
		+	====Subclasses====
		+	[[Semirings with identity and zero]]
		+
		+	====Superclasses====
		+
		+	====References====
		+
		+	[(Ln19xx>
		+	)]