1639, 1641, and 1643 are semiprimes (A092125).
1639 is a nonagonal number (A001106).
1639 is the number of binary rooted trees with 16 vertices.
1639 is 2020201 in base 3 and 121213 in base 4. It is 2221 in base 9.
Every base 6 digit of 1639 is a base 10 digit of 1639: 11331 (A037401).
1639 is the sum of two pentagonal numbers in exactly two different ways (A064826).
Source: What's Special About This Number?
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