It is a happy number.
921 is an esthetic number in base 4, because in such base its adjacent digits differ by 1.
It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 129 = 3 ⋅43.
It is a D-number.
It is an alternating number because its digits alternate between odd and even.
It is a Duffinian number.
It is a plaindrome in base 9, base 13, base 14 and base 16.
It is a nialpdrome in base 10.
It is an amenable number.
921 is a wasteful number, since it uses less digits than its factorization.
921 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 310.
The square root of 921 is about 30.3479818110. The cubic root of 921 is about 9.7294108588.
Subtracting from 921 its sum of digits (12), we obtain a palindrome (909).
Adding to 921 its product of digits (18), we get a palindrome (939).
Subtracting from 921 its product of digits (18), we obtain a triangular number (903 = T42).
The spelling of 921 in words is "nine hundred twenty-one", and thus it is an aban number.