21210661 has 4 divisors (see below), whose sum is σ = 21501292. Its totient is φ = 20920032.

The previous prime is 21210649. The next prime is 21210691. The reversal of 21210661 is 16601212.

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 18438436 + 2772225 = 4294^2 + 1665^2 .

It is a cyclic number.

It is not a de Polignac number, because 21210661 - 2¹¹ = 21208613 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 (21210691) 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, 145206 + ... + 145351.

It is an arithmetic number, because the mean of its divisors is an integer number (5375323).

Almost surely, 2^21210661 is an apocalyptic number.

It is an amenable number.

21210661 is a deficient number, since it is larger than the sum of its proper divisors (290631).

21210661 is an equidigital number, since it uses as much as digits as its factorization.

21210661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 290630.

The product of its (nonzero) digits is 144, while the sum is 19.

The square root of 21210661 is about 4605.5033383985. The cubic root of 21210661 is about 276.8118859890.

Adding to 21210661 its reverse (16601212), we get a palindrome (37811873).

The spelling of 21210661 in words is "twenty-one million, two hundred ten thousand, six hundred sixty-one".