21520721 has 2 divisors, whose sum is σ = 21520722. Its totient is φ = 21520720.

The previous prime is 21520699. The next prime is 21520729. The reversal of 21520721 is 12702512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21520321 + 400 = 4639^2 + 20^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21520721 is a prime.

It is a Chen prime.

It is equal to p_1360811 and since 21520721 and 1360811 have the same sum of digits, it is a Honaker prime.

It is a junction number, because it is equal to n+sod(n) for n = 21520693 and 21520702.

It is not a weakly prime, because it can be changed into another prime (21520729) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10760360 + 10760361.

It is an arithmetic number, because the mean of its divisors is an integer number (10760361).

Almost surely, 2^21520721 is an apocalyptic number.

It is an amenable number.

21520721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21520721 is an equidigital number, since it uses as much as digits as its factorization.

21520721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 280, while the sum is 20.

The square root of 21520721 is about 4639.0431125395. The cubic root of 21520721 is about 278.1541896057.

The spelling of 21520721 in words is "twenty-one million, five hundred twenty thousand, seven hundred twenty-one".