21558661 has 8 divisors (see below), whose sum is σ = 22239000. Its totient is φ = 20885760.

The previous prime is 21558643. The next prime is 21558689. The reversal of 21558661 is 16685512.

It is a happy number.

It can be written as a sum of positive squares in 4 ways, for example, as 2461761 + 19096900 = 1569^2 + 4370^2 .

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

It is a cyclic number.

It is not a de Polignac number, because 21558661 - 2⁹ = 21558149 is a prime.

It is a super-2 number, since 2×21558661² = 929551728225842, 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 (21558961) 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, 4345 + ... + 7873.

It is an arithmetic number, because the mean of its divisors is an integer number (2779875).

Almost surely, 2^21558661 is an apocalyptic number.

It is an amenable number.

21558661 is a deficient number, since it is larger than the sum of its proper divisors (680339).

21558661 is a wasteful number, since it uses less digits than its factorization.

21558661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3719.

The product of its digits is 14400, while the sum is 34.

The square root of 21558661 is about 4643.1305172265. The cubic root of 21558661 is about 278.3175511468.

It can be divided in two parts, 2155 and 8661, that added together give a square (10816 = 104²).

The spelling of 21558661 in words is "twenty-one million, five hundred fifty-eight thousand, six hundred sixty-one".