91 is: the twenty-seventh distinct semiprime and the second of the form (7.q), where q is a higher prime. the aliquot sum of 91 is 21 33; itself a semiprime, within an aliquot sequence of two composite numbers (91,21,11, 1,0) to the prime in the 11-aliquot tree. 91 is the fourth composite number in the 11-aliquot tree.(91,51,21,18). the 13th triangular number. a hexagonal number,one of the few such numbers to also be a centered hexagonal number. a centered nonagonal number. a centered cube number. a square pyramidal number, being the sum of the squares of the first six integers. the smallest positive integer expressible as a sum of two cubes in two different ways if negative roots are allowed (alternatively the sum of two cubes and the difference of two cubes): 91 = 63 + (−5)3 = 43 + 33.
