21561661 has 8 divisors (see below), whose sum is σ = 24905664. Its totient is φ = 18448320.

The previous prime is 21561629. The next prime is 21561697. The reversal of 21561661 is 16616512.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21561661 - 2⁵ = 21561629 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 (21561601) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 57465 + ... + 57838.

It is an arithmetic number, because the mean of its divisors is an integer number (3113208).

Almost surely, 2^21561661 is an apocalyptic number.

It is an amenable number.

21561661 is a deficient number, since it is larger than the sum of its proper divisors (3344003).

21561661 is a wasteful number, since it uses less digits than its factorization.

21561661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 115331.

The product of its digits is 2160, while the sum is 28.

The square root of 21561661 is about 4643.4535638897. The cubic root of 21561661 is about 278.3304603271.

It can be divided in two parts, 21561 and 661, that added together give a palindrome (22222).

The spelling of 21561661 in words is "twenty-one million, five hundred sixty-one thousand, six hundred sixty-one".