21720661 has 4 divisors (see below), whose sum is σ = 21785452. Its totient is φ = 21655872.

The previous prime is 21720649. The next prime is 21720689. The reversal of 21720661 is 16602712.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 1739761 + 19980900 = 1319^2 + 4470^2 .

It is a cyclic number.

It is not a de Polignac number, because 21720661 - 2¹³ = 21712469 is a prime.

It is a super-2 number, since 2×21720661² = 943574228553842, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21720631) 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, 31890 + ... + 32563.

It is an arithmetic number, because the mean of its divisors is an integer number (5446363).

Almost surely, 2^21720661 is an apocalyptic number.

It is an amenable number.

21720661 is a deficient number, since it is larger than the sum of its proper divisors (64791).

21720661 is an equidigital number, since it uses as much as digits as its factorization.

21720661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 64790.

The product of its (nonzero) digits is 1008, while the sum is 25.

The square root of 21720661 is about 4660.5429941156. The cubic root of 21720661 is about 279.0129403093.

The spelling of 21720661 in words is "twenty-one million, seven hundred twenty thousand, six hundred sixty-one".