21355721 has 4 divisors (see below), whose sum is σ = 21365700. Its totient is φ = 21345744.

The previous prime is 21355717. The next prime is 21355729. The reversal of 21355721 is 12755312.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 1597696 + 19758025 = 1264^2 + 4445^2 .

It is a cyclic number.

It is not a de Polignac number, because 21355721 - 2² = 21355717 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21355729) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 326 + ... + 6543.

It is an arithmetic number, because the mean of its divisors is an integer number (5341425).

Almost surely, 2^21355721 is an apocalyptic number.

It is an amenable number.

21355721 is a deficient number, since it is larger than the sum of its proper divisors (9979).

21355721 is an equidigital number, since it uses as much as digits as its factorization.

21355721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9978.

The product of its digits is 2100, while the sum is 26.

The square root of 21355721 is about 4621.2250540306. The cubic root of 21355721 is about 277.4414929545.

The spelling of 21355721 in words is "twenty-one million, three hundred fifty-five thousand, seven hundred twenty-one".