|
19 (nineteen) is the natural number following 18 and preceding 20. It is a prime number. In English speech, the numbers 19 and 90 are often confused, as sounding very similar. When carefully enunciated, they differ in which syllable is stressed: 19 vs 90 . However, in dates such as 1999, and when contrasting numbers in the teens and when counting, such as ''17, 18, 19,'' the stress shifts to the first syllable: 19 . == Mathematics == 19 is the 8th prime number. The sequence continues 23, 29, 31, 37, ... . 19 is the seventh Mersenne prime exponent. 19 is the aliquot sum of two odd discrete semiprimes, 65 and 77 and is the base of the 19-aliquot tree. 19 is the lowest prime centered triangular number, a centered hexagonal number and a Heegner number. The only non-trivial normal magic hexagon contains 19 hexagons (the other being 1). 19 is the first number with more than one digit that can be written from base 2 to base 19 using only the digits 0 to 9; the other number is 20.〔Sequence A131646 in The On-Line Encyclopedia of Integer Sequences〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「19 (number)」の詳細全文を読む スポンサード リンク
|