866721781 has 4 divisors (see below), whose sum is σ = 890146732. Its totient is φ = 843296832.

The previous prime is 866721769. The next prime is 866721799. The reversal of 866721781 is 187127668.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 497736100 + 368985681 = 22310^2 + 19209^2 .

It is a cyclic number.

It is not a de Polignac number, because 866721781 - 2⁵ = 866721749 is a prime.

It is a super-2 number, since 2×866721781² = 1502413291319623922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (866721731) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 11712420 + ... + 11712493.

It is an arithmetic number, because the mean of its divisors is an integer number (222536683).

Almost surely, 2^866721781 is an apocalyptic number.

It is an amenable number.

866721781 is a deficient number, since it is larger than the sum of its proper divisors (23424951).

866721781 is a wasteful number, since it uses less digits than its factorization.

866721781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 23424950.

The product of its digits is 225792, while the sum is 46.

The square root of 866721781 is about 29440.1389432863. The cubic root of 866721781 is about 953.4397119815.

The spelling of 866721781 in words is "eight hundred sixty-six million, seven hundred twenty-one thousand, seven hundred eighty-one".