21991001 has 2 divisors, whose sum is σ = 21991002. Its totient is φ = 21991000.

The previous prime is 21990989. The next prime is 21991027. The reversal of 21991001 is 10019912.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21864976 + 126025 = 4676^2 + 355^2 .

It is a cyclic number.

It is not a de Polignac number, because 21991001 - 2¹⁸ = 21728857 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21991091) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10995500 + 10995501.

It is an arithmetic number, because the mean of its divisors is an integer number (10995501).

Almost surely, 2^21991001 is an apocalyptic number.

It is an amenable number.

21991001 is a deficient number, since it is larger than the sum of its proper divisors (1).

21991001 is an equidigital number, since it uses as much as digits as its factorization.

21991001 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 162, while the sum is 23.

The square root of 21991001 is about 4689.4563650811. The cubic root of 21991001 is about 280.1657224734.

The spelling of 21991001 in words is "twenty-one million, nine hundred ninety-one thousand, one".