686521981 has 2 divisors, whose sum is σ = 686521982. Its totient is φ = 686521980.

The previous prime is 686521949. The next prime is 686521991. The reversal of 686521981 is 189125686.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 426216025 + 260305956 = 20645^2 + 16134^2 .

It is a cyclic number.

It is not a de Polignac number, because 686521981 - 2⁵ = 686521949 is a prime.

It is a super-2 number, since 2×686521981² = 942624860792328722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (686521921) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 343260990 + 343260991.

It is an arithmetic number, because the mean of its divisors is an integer number (343260991).

Almost surely, 2^686521981 is an apocalyptic number.

It is an amenable number.

686521981 is a deficient number, since it is larger than the sum of its proper divisors (1).

686521981 is an equidigital number, since it uses as much as digits as its factorization.

686521981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 207360, while the sum is 46.

The square root of 686521981 is about 26201.5644761911. The cubic root of 686521981 is about 882.1683703399.

The spelling of 686521981 in words is "six hundred eighty-six million, five hundred twenty-one thousand, nine hundred eighty-one".