6661906021 has 4 divisors (see below), whose sum is σ = 6816834112. Its totient is φ = 6506977932.

The previous prime is 6661905977. The next prime is 6661906037. The reversal of 6661906021 is 1206091666.

6661906021 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 6661906021 - 2²⁵ = 6628351589 is a prime.

It is the 81621-st Hogben number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (6661906081) by changing a digit.

It is a nontrivial repunit in base 81620.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 77463981 + ... + 77464066.

It is an arithmetic number, because the mean of its divisors is an integer number (1704208528).

Almost surely, 2^6661906021 is an apocalyptic number.

It is an amenable number.

6661906021 is a deficient number, since it is larger than the sum of its proper divisors (154928091).

6661906021 is a wasteful number, since it uses less digits than its factorization.

6661906021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 154928090.

The product of its (nonzero) digits is 23328, while the sum is 37.

The square root of 6661906021 is about 81620.5000045944. The cubic root of 6661906021 is about 1881.6239571734.

Adding to 6661906021 its reverse (1206091666), we get a palindrome (7867997687).

The spelling of 6661906021 in words is "six billion, six hundred sixty-one million, nine hundred six thousand, twenty-one".