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Mathematics. 15 is a lucky number.; a triangular number.; a hexagonal number.; a pentatope number.; along with 13, one of two numbers within the teen numerical range (1319) not to use a singledigit number preceding the teen suffix; instead, it uses the adjective form of five (fifth) as the predecessor of said teen suffix.; the 5th bell number (i.e. the number of partitions for a set of size 4).
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