| Number: | 55 |
| Divisor: | 1, 5, 11, 55 |
55 (fifty-five) is the natural number following 54 and preceding 56.
55 is the fifteenth discrete semiprime,[1] and the second with 5 as the lowest non-unitary factor. Thus, of the form 5 × q with q a higher prime, in this case equal to 11.
It contains an aliquot sum of 17; the seventh prime number, within an aliquot sequence of one composite number (55, 17, 1, 0) that is rooted in the 17-aliquot tree.
55 is the tenth Fibonacci number.[2] It is the largest Fibonacci number to also be a triangular number (the tenth as well);[3] it is furthermore the fourth doubly triangular number.[4]
55 is also an early member inside other families of polygonal numbers; it is strictly (when including 0 as the zeroth indexed member) the fifth:
It is also the fourth centered nonagonal number,[5] and the third centered icosahedral number.[6]
In decimal, 55 is a Kaprekar number,[7] whose digit sum is also 10. It is the first number to be a sum of more than one pair of numbers which mirror each other (23 + 32 and 14 + 41).