668216781 has 12 divisors (see below), whose sum is σ = 991289052. Its totient is φ = 433437696.

The previous prime is 668216779. The next prime is 668216789. The reversal of 668216781 is 187612866.

It is a happy number.

It can be written as a sum of positive squares in 2 ways, for example, as 554602500 + 113614281 = 23550^2 + 10659^2 .

It is not a de Polignac number, because 668216781 - 2¹ = 668216779 is a prime.

It is a super-2 number, since 2×668216781² = 893027332820003922, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (668216789) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 1002996 + ... + 1003661.

It is an arithmetic number, because the mean of its divisors is an integer number (82607421).

Almost surely, 2^668216781 is an apocalyptic number.

It is an amenable number.

668216781 is a deficient number, since it is larger than the sum of its proper divisors (323072271).

668216781 is a wasteful number, since it uses less digits than its factorization.

668216781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2006700 (or 2006697 counting only the distinct ones).

The product of its digits is 193536, while the sum is 45.

The square root of 668216781 is about 25849.8893808078. The cubic root of 668216781 is about 874.2570154413.

The spelling of 668216781 in words is "six hundred sixty-eight million, two hundred sixteen thousand, seven hundred eighty-one".