21147721 has 8 divisors (see below), whose sum is σ = 24203840. Its totient is φ = 18100368.

The previous prime is 21147713. The next prime is 21147727. The reversal of 21147721 is 12774112.

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 21147721 - 2³ = 21147713 is a prime.

It is a super-2 number, since 2×21147721² = 894452206987682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21147727) 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, 4255 + ... + 7771.

It is an arithmetic number, because the mean of its divisors is an integer number (3025480).

Almost surely, 2^21147721 is an apocalyptic number.

It is an amenable number.

21147721 is a deficient number, since it is larger than the sum of its proper divisors (3056119).

21147721 is an equidigital number, since it uses as much as digits as its factorization.

21147721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4383.

The product of its digits is 784, while the sum is 25.

The square root of 21147721 is about 4598.6651324053. The cubic root of 21147721 is about 276.5378130964.

The spelling of 21147721 in words is "twenty-one million, one hundred forty-seven thousand, seven hundred twenty-one".