| 21. | For another example, consider again the relation | on natural numbers.
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| 22. | As each natural number has a unique multiplicative decomposition into powers of primes.
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| 23. | Second-order arithmetic directly formalizes natural numbers and sets of natural numbers.
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| 24. | The digital root is an interesting property of a natural number.
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| 25. | In this sense, prime numbers occur more often than squares of natural numbers.
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| 26. | The set of finite lists of natural numbers is the initial such algebra.
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| 27. | Other independence results concern Peano arithmetic and other formalizations of the natural numbers.
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| 28. | These three properties were originally included among the Peano axioms for natural numbers.
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| 29. | And I would like to have categorical definition of natural numbers.
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| 30. | A common example of this is the cross product of positive natural numbers.
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