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1. The method of proving that something exists by showing how to create it is called proof by constructive algorithm.   
2.  
   ¡ A recursive definition is characteristically a three-step process.

     First, we specify some basic objects in the set.

     Second, we give rules for constructing more objects in the set from the ones we already know.

     Third, we declare  that no objects except those constructed in this way are allowed in the set.

    

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