21561650021 has 4 divisors (see below), whose sum is σ = 21564464844. Its totient is φ = 21558835200.

The previous prime is 21561650009. The next prime is 21561650029. The reversal of 21561650021 is 12005616512.

It is a happy number.

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 15502740100 + 6058909921 = 124510^2 + 77839^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21561650021 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21561650029) 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, 1395890 + ... + 1411251.

It is an arithmetic number, because the mean of its divisors is an integer number (5391116211).

Almost surely, 2^21561650021 is an apocalyptic number.

It is an amenable number.

21561650021 is a deficient number, since it is larger than the sum of its proper divisors (2814823).

21561650021 is an equidigital number, since it uses as much as digits as its factorization.

21561650021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2814822.

The product of its (nonzero) digits is 3600, while the sum is 29.

The spelling of 21561650021 in words is "twenty-one billion, five hundred sixty-one million, six hundred fifty thousand, twenty-one".