Forty-five is a triangular number, and in particular the sum of all the decimal digits (0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45). It is the smallest triangle number (after 1) which can be written as the sum of two squares. It is also a hexagonal and 16-gonal number.

45 is the sixth positive integer with a prime factorization of the form p²q, with p and q being prime.

33 is the aliquot sum of 45 and the aliquot sequence of 45 is (45, 33, 15, 9, 4, 3, 1, 0).

Since the greatest prime factor of 45² + 1 = 2026 is 1013, which is much more than 45 twice, 45 is a Størmer number.

In base 10, it is a Kaprekar number and a Harshad number.

Forty-five may also refer to:

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