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In this chapter, we focus on the language aspect of logic; the reasoning aspect is investigated in Appendix Proving. A third aspect, the automatization of problem solving on computers, is discussed in another course "Logic for Computer Science".  
Logical constants can be considered as connectives of arity 0, i.e., as connectives that do not take any arguments.  
See the followup comments on the definition of "union" and "powerset".  
This does not contradict the statement that for every positive n in N, there is a unique pair -n in Z and +n in Z. Rather it shows us that the union of two disjoint copies of an infinite set S still has the same size as S, i.e., that our intuition of set sizes cannot be automatically applied to the infinite case.  
In computer science, trees grow downwards! ;-)  
The corresponding propositions may be nevertheless true.

Author: Wolfgang Schreiner
Last Modification: October 4, 1999

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