The purpose of this paper is to consider the question of how we can represent numbers (especially natural numbers) and how our choice of a representation affects our ability to compute various functions. In particular, we show the importance of computability of the characteristic function of identity in a representation of numbers. It turns out that it is a very strong assumption that significantly increases the scope of our knowledge about a given representation, including our abilityto tell which functions are computable in this representation.
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