21811721 has 2 divisors, whose sum is σ = 21811722. Its totient is φ = 21811720.

The previous prime is 21811717. The next prime is 21811723. The reversal of 21811721 is 12711812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21492496 + 319225 = 4636^2 + 565^2 .

It is a cyclic number.

It is not a de Polignac number, because 21811721 - 2² = 21811717 is a prime.

It is a Sophie Germain prime.

Together with 21811723, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21811723) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10905860 + 10905861.

It is an arithmetic number, because the mean of its divisors is an integer number (10905861).

Almost surely, 2^21811721 is an apocalyptic number.

It is an amenable number.

21811721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21811721 is an equidigital number, since it uses as much as digits as its factorization.

21811721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 224, while the sum is 23.

The square root of 21811721 is about 4670.3020244948. The cubic root of 21811721 is about 279.4023007151.

Subtracting from 21811721 its reverse (12711812), we obtain a palindrome (9099909).

The spelling of 21811721 in words is "twenty-one million, eight hundred eleven thousand, seven hundred twenty-one".