21721681 has 4 divisors (see below), whose sum is σ = 21983472. Its totient is φ = 21459892.

The previous prime is 21721621. The next prime is 21721717. The reversal of 21721681 is 18612712.

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 a de Polignac number, because none of the positive numbers 2^k-21721681 is a prime.

It is a super-2 number, since 2×21721681² = 943662850931522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21721621) 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, 130771 + ... + 130936.

It is an arithmetic number, because the mean of its divisors is an integer number (5495868).

Almost surely, 2^21721681 is an apocalyptic number.

It is an amenable number.

21721681 is a deficient number, since it is larger than the sum of its proper divisors (261791).

21721681 is an equidigital number, since it uses as much as digits as its factorization.

21721681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 261790.

The product of its digits is 1344, while the sum is 28.

The square root of 21721681 is about 4660.6524221401. The cubic root of 21721681 is about 279.0173077139.

The spelling of 21721681 in words is "twenty-one million, seven hundred twenty-one thousand, six hundred eighty-one".