109 is the 29th prime number, so it is a prime with a prime subscript. The previous prime is 107, making them both twin primes. 109 is a centered triangular number.

There are exactly 109 different families of subsets of a three-element set whose union includes all three elements, 109 different loops (invertible but not necessarily associative binary operations with an identity) on six elements. and 109 squares on an infinite chessboard that can be reached by a knight within three moves.

109 is also the atomic number of meitnerium.

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