21356721 has 12 divisors (see below), whose sum is σ = 31431348. Its totient is φ = 13968864.

The previous prime is 21356711. The next prime is 21356723. The reversal of 21356721 is 12765312.

It is a happy number.

It can be written as a sum of positive squares in 2 ways, for example, as 14930496 + 6426225 = 3864^2 + 2535^2 .

It is not a de Polignac number, because 21356721 - 2⁵ = 21356689 is a prime.

It is a super-2 number, since 2×21356721² = 912219063743682, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21356723) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 21910 + ... + 22863.

It is an arithmetic number, because the mean of its divisors is an integer number (2619279).

Almost surely, 2^21356721 is an apocalyptic number.

It is an amenable number.

21356721 is a deficient number, since it is larger than the sum of its proper divisors (10074627).

21356721 is a wasteful number, since it uses less digits than its factorization.

21356721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 44832 (or 44829 counting only the distinct ones).

The product of its digits is 2520, while the sum is 27.

The square root of 21356721 is about 4621.3332491825. Note that the first 3 decimals coincide. The cubic root of 21356721 is about 277.4458233658.

The spelling of 21356721 in words is "twenty-one million, three hundred fifty-six thousand, seven hundred twenty-one".