666781 has 4 divisors (see below), whose sum is σ = 670464. Its totient is φ = 663100.

The previous prime is 666773. The next prime is 666811. The reversal of 666781 is 187666.

666781 is digitally balanced in base 2, 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 666781 - 2³ = 666773 is a prime.

It is a super-2 number, since 2×666781² = 889193803922, which contains 22 as substring.

It is a Duffinian number.

666781 is a lucky number.

It is a plaindrome in base 9.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (666751) 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, 1555 + ... + 1936.

It is an arithmetic number, because the mean of its divisors is an integer number (167616).

2⁶⁶⁶⁷⁸¹ is an apocalyptic number.

It is an amenable number.

666781 is a deficient number, since it is larger than the sum of its proper divisors (3683).

666781 is a wasteful number, since it uses less digits than its factorization.

666781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3682.

The product of its digits is 12096, while the sum is 34.

The square root of 666781 is about 816.5665925079. The cubic root of 666781 is about 87.3630401565.

The spelling of 666781 in words is "six hundred sixty-six thousand, seven hundred eighty-one".