661664021 has 8 divisors (see below), whose sum is σ = 701017920. Its totient is φ = 622358208.

The previous prime is 661663997. The next prime is 661664053. The reversal of 661664021 is 120466166.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 661664021 - 2⁶ = 661663957 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 (661664071) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 18560 + ... + 40838.

It is an arithmetic number, because the mean of its divisors is an integer number (87627240).

Almost surely, 2^661664021 is an apocalyptic number.

It is an amenable number.

661664021 is a deficient number, since it is larger than the sum of its proper divisors (39353899).

661664021 is a wasteful number, since it uses less digits than its factorization.

661664021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 24043.

The product of its (nonzero) digits is 10368, while the sum is 32.

The square root of 661664021 is about 25722.8307345828. The cubic root of 661664021 is about 871.3898694154.

The spelling of 661664021 in words is "six hundred sixty-one million, six hundred sixty-four thousand, twenty-one".