21691721 has 2 divisors, whose sum is σ = 21691722. Its totient is φ = 21691720.

The previous prime is 21691693. The next prime is 21691751. The reversal of 21691721 is 12719612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 16516096 + 5175625 = 4064^2 + 2275^2 .

It is a cyclic number.

It is not a de Polignac number, because 21691721 - 2¹⁴ = 21675337 is a prime.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21691751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10845860 + 10845861.

It is an arithmetic number, because the mean of its divisors is an integer number (10845861).

Almost surely, 2^21691721 is an apocalyptic number.

It is an amenable number.

21691721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21691721 is an equidigital number, since it uses as much as digits as its factorization.

21691721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1512, while the sum is 29.

The square root of 21691721 is about 4657.4371708054. The cubic root of 21691721 is about 278.8889688935. Note that the first 3 decimals are identical.

The spelling of 21691721 in words is "twenty-one million, six hundred ninety-one thousand, seven hundred twenty-one".