668781821 has 4 divisors (see below), whose sum is σ = 669647772. Its totient is φ = 667915872.

The previous prime is 668781811. The next prime is 668781829. The reversal of 668781821 is 128187866.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 128187866 = 2 ⋅64093933.

It can be written as a sum of positive squares in 2 ways, for example, as 261307225 + 407474596 = 16165^2 + 20186^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-668781821 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 (668781829) 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, 431816 + ... + 433361.

It is an arithmetic number, because the mean of its divisors is an integer number (167411943).

Almost surely, 2^668781821 is an apocalyptic number.

It is an amenable number.

668781821 is a deficient number, since it is larger than the sum of its proper divisors (865951).

668781821 is an equidigital number, since it uses as much as digits as its factorization.

668781821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 865950.

The product of its digits is 258048, while the sum is 47.

The square root of 668781821 is about 25860.8163250892. The cubic root of 668781821 is about 874.5033681346.

The spelling of 668781821 in words is "six hundred sixty-eight million, seven hundred eighty-one thousand, eight hundred twenty-one".