21561721 has 4 divisors (see below), whose sum is σ = 21603472. Its totient is φ = 21519972.

The previous prime is 21561697. The next prime is 21561733. The reversal of 21561721 is 12716512.

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 21561721 - 2¹³ = 21553529 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21561781) 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, 20091 + ... + 21136.

It is an arithmetic number, because the mean of its divisors is an integer number (5400868).

Almost surely, 2^21561721 is an apocalyptic number.

It is an amenable number.

21561721 is a deficient number, since it is larger than the sum of its proper divisors (41751).

21561721 is an equidigital number, since it uses as much as digits as its factorization.

21561721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 41750.

The product of its digits is 840, while the sum is 25.

The square root of 21561721 is about 4643.4600245937. The cubic root of 21561721 is about 278.3307184985.

The spelling of 21561721 in words is "twenty-one million, five hundred sixty-one thousand, seven hundred twenty-one".