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; 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): . (See
1729 for more details). This implies that 91 is the second
cabtaxi number. • the smallest positive integer expressible as a sum of six distinct squares: . • The only other ways to write 91 as a sum of distinct squares are: and . • the smallest
pseudoprime satisfying the congruence . • a
repdigit in base 9 (1119). • palindromic in bases 3 (101013), 9 (1119), and 12 (7712). • a Riordan number. • the smallest number that looks
prime but is not, proven using the Rotten Theorem by
John Conway. The decimal equivalent of the fraction can be obtained by using powers of 9. ==In science==