21667661 has 4 divisors (see below), whose sum is σ = 21678912. Its totient is φ = 21656412.

The previous prime is 21667651. The next prime is 21667703. The reversal of 21667661 is 16676612.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21667661 - 2¹⁰ = 21666637 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21667651) 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, 1925 + ... + 6858.

It is an arithmetic number, because the mean of its divisors is an integer number (5419728).

Almost surely, 2^21667661 is an apocalyptic number.

It is an amenable number.

21667661 is a deficient number, since it is larger than the sum of its proper divisors (11251).

21667661 is an equidigital number, since it uses as much as digits as its factorization.

21667661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 11250.

The product of its digits is 18144, while the sum is 35.

The square root of 21667661 is about 4654.8534885644. The cubic root of 21667661 is about 278.7858181567.

The spelling of 21667661 in words is "twenty-one million, six hundred sixty-seven thousand, six hundred sixty-one".