16821991 has 2 divisors, whose sum is σ = 16821992. Its totient is φ = 16821990.

The previous prime is 16821989. The next prime is 16822037. The reversal of 16821991 is 19912861.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 16821991 - 2¹ = 16821989 is a prime.

Together with 16821989, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 16821991.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (16821191) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 8410995 + 8410996.

It is an arithmetic number, because the mean of its divisors is an integer number (8410996).

Almost surely, 2^16821991 is an apocalyptic number.

16821991 is a deficient number, since it is larger than the sum of its proper divisors (1).

16821991 is an equidigital number, since it uses as much as digits as its factorization.

16821991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 7776, while the sum is 37.

The square root of 16821991 is about 4101.4620563892. The cubic root of 16821991 is about 256.2275351315.

The spelling of 16821991 in words is "sixteen million, eight hundred twenty-one thousand, nine hundred ninety-one".