21212561 has 4 divisors (see below), whose sum is σ = 21612852. Its totient is φ = 20812272.

The previous prime is 21212537. The next prime is 21212591. The reversal of 21212561 is 16521212.

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 6150400 + 15062161 = 2480^2 + 3881^2 .

It is a cyclic number.

It is not a de Polignac number, because 21212561 - 2⁶ = 21212497 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21212501) 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, 200066 + ... + 200171.

It is an arithmetic number, because the mean of its divisors is an integer number (5403213).

Almost surely, 2^21212561 is an apocalyptic number.

It is an amenable number.

21212561 is a deficient number, since it is larger than the sum of its proper divisors (400291).

21212561 is an equidigital number, since it uses as much as digits as its factorization.

21212561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 400290.

The product of its digits is 240, while the sum is 20.

The square root of 21212561 is about 4605.7096087357. The cubic root of 21212561 is about 276.8201511233.

Adding to 21212561 its reverse (16521212), we get a palindrome (37733773).

The spelling of 21212561 in words is "twenty-one million, two hundred twelve thousand, five hundred sixty-one".