21010721 has 2 divisors, whose sum is σ = 21010722. Its totient is φ = 21010720.

The previous prime is 21010711. The next prime is 21010723. The reversal of 21010721 is 12701012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20830096 + 180625 = 4564^2 + 425^2 .

It is a cyclic number.

It is not a de Polignac number, because 21010721 - 2¹⁰ = 21009697 is a prime.

Together with 21010723, it forms a pair of twin primes.

It is a Chen prime.

It is an alternating number because its digits alternate between even and odd.

It is a junction number, because it is equal to n+sod(n) for n = 21010696 and 21010705.

It is not a weakly prime, because it can be changed into another prime (21010723) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10505360 + 10505361.

It is an arithmetic number, because the mean of its divisors is an integer number (10505361).

Almost surely, 2^21010721 is an apocalyptic number.

It is an amenable number.

21010721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21010721 is an equidigital number, since it uses as much as digits as its factorization.

21010721 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 28, while the sum is 14.

The square root of 21010721 is about 4583.7453026973. The cubic root of 21010721 is about 275.9393595334.

Adding to 21010721 its reverse (12701012), we get a palindrome (33711733).

The spelling of 21010721 in words is "twenty-one million, ten thousand, seven hundred twenty-one".