19 is a centered triangular number. Nineteen is the eighth prime number. Number theory. 19 forms a twin prime with 17, [1] a cousin prime with 23, [2] and a sexy prime with 13. [3] 19 is the fifth central trinomial coefficient, [4] and the maximum number of fourth powers needed to sum up to any natural number (see, Waring's problem). [5] . More @Wikipedia
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