Mathematics
55 is:
- the 10th Fibonacci number [1] and the 10th triangular number, [2] The sum of 55's digits is also 10.
- the 5th heptagonal number, [3] the 5th square pyramidal number, [4] and the 4th centered nonagonal number. [5]
55 is also the 19th semiprime and the 15th squarefree semiprime, [6] [7] as well as the 32nd nontotient number (including odd numbers > 1) [8] the 36th arithmetic number, [9] the 38th composite number [10] (where the abundance of 55 = - 38), [11] the 43rd deficient number, [12] the 45th trapezoidal number, [13] and the 49th polite number. [14]