21306661 has 4 divisors (see below), whose sum is σ = 22560012. Its totient is φ = 20053312.

The previous prime is 21306617. The next prime is 21306707. The reversal of 21306661 is 16660312.

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 13184161 + 8122500 = 3631^2 + 2850^2 .

It is a cyclic number.

It is not a de Polignac number, because 21306661 - 2⁹ = 21306149 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 (21316661) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 626650 + ... + 626683.

It is an arithmetic number, because the mean of its divisors is an integer number (5640003).

Almost surely, 2^21306661 is an apocalyptic number.

It is an amenable number.

21306661 is a deficient number, since it is larger than the sum of its proper divisors (1253351).

21306661 is a wasteful number, since it uses less digits than its factorization.

21306661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1253350.

The product of its (nonzero) digits is 1296, while the sum is 25.

The square root of 21306661 is about 4615.9138856785. The cubic root of 21306661 is about 277.2288767673.

Adding to 21306661 its reverse (16660312), we get a palindrome (37966973).

The spelling of 21306661 in words is "twenty-one million, three hundred six thousand, six hundred sixty-one".