1668900721 has 4 divisors (see below), whose sum is σ = 1684498032. Its totient is φ = 1653303412.

The previous prime is 1668900719. The next prime is 1668900731. The reversal of 1668900721 is 1270098661.

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, and also an emirpimes, since its reverse is a distinct semiprime: 1270098661 = 13 ⋅97699897.

It is a cyclic number.

It is not a de Polignac number, because 1668900721 - 2¹ = 1668900719 is a prime.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1668900731) 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, 7798495 + ... + 7798708.

It is an arithmetic number, because the mean of its divisors is an integer number (421124508).

Almost surely, 2^1668900721 is an apocalyptic number.

It is an amenable number.

1668900721 is a deficient number, since it is larger than the sum of its proper divisors (15597311).

1668900721 is a wasteful number, since it uses less digits than its factorization.

1668900721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 15597310.

The product of its (nonzero) digits is 36288, while the sum is 40.

The square root of 1668900721 is about 40852.1813493478. The cubic root of 1668900721 is about 1186.1606178335.

The spelling of 1668900721 in words is "one billion, six hundred sixty-eight million, nine hundred thousand, seven hundred twenty-one".