21510721 has 8 divisors (see below), whose sum is σ = 22314600. Its totient is φ = 20711040.

The previous prime is 21510719. The next prime is 21510733. The reversal of 21510721 is 12701512.

It can be written as a sum of positive squares in 4 ways, for example, as 7485696 + 14025025 = 2736^2 + 3745^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21510721 - 2¹ = 21510719 is a prime.

It is a super-2 number, since 2×21510721² = 925422235879682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21510121) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 12565 + ... + 14173.

It is an arithmetic number, because the mean of its divisors is an integer number (2789325).

Almost surely, 2^21510721 is an apocalyptic number.

It is an amenable number.

21510721 is a deficient number, since it is larger than the sum of its proper divisors (803879).

21510721 is a wasteful number, since it uses less digits than its factorization.

21510721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2099.

The product of its (nonzero) digits is 140, while the sum is 19.

The square root of 21510721 is about 4637.9651788257. The cubic root of 21510721 is about 278.1110997724. Note that the first 3 decimals are identical.

The spelling of 21510721 in words is "twenty-one million, five hundred ten thousand, seven hundred twenty-one".