21506721 has 4 divisors (see below), whose sum is σ = 28675632. Its totient is φ = 14337812.

The previous prime is 21506707. The next prime is 21506729. The reversal of 21506721 is 12760512.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21506721 - 2⁷ = 21506593 is a prime.

It is a super-3 number, since 3×21506721³ (a number of 23 digits) contains 333 as substring.

It is a junction number, because it is equal to n+sod(n) for n = 21506691 and 21506700.

It is not an unprimeable number, because it can be changed into a prime (21506729) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3584451 + ... + 3584456.

It is an arithmetic number, because the mean of its divisors is an integer number (7168908).

Almost surely, 2^21506721 is an apocalyptic number.

It is an amenable number.

21506721 is a deficient number, since it is larger than the sum of its proper divisors (7168911).

21506721 is an equidigital number, since it uses as much as digits as its factorization.

21506721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7168910.

The product of its (nonzero) digits is 840, while the sum is 24.

The square root of 21506721 is about 4637.5339351858. The cubic root of 21506721 is about 278.0938600998.

The spelling of 21506721 in words is "twenty-one million, five hundred six thousand, seven hundred twenty-one".