665981 has 2 divisors, whose sum is σ = 665982. Its totient is φ = 665980.

The previous prime is 665953. The next prime is 665983. The reversal of 665981 is 189566.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 648025 + 17956 = 805^2 + 134^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-665981 is a prime.

It is a super-2 number, since 2×665981² = 887061384722, which contains 22 as substring.

Together with 665983, it forms a pair of twin primes.

It is a Chen prime.

It is an Ulam number.

It is a plaindrome in base 9.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (665983) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 332990 + 332991.

It is an arithmetic number, because the mean of its divisors is an integer number (332991).

2⁶⁶⁵⁹⁸¹ is an apocalyptic number.

It is an amenable number.

665981 is a deficient number, since it is larger than the sum of its proper divisors (1).

665981 is an equidigital number, since it uses as much as digits as its factorization.

665981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 12960, while the sum is 35.

The square root of 665981 is about 816.0765895429. The cubic root of 665981 is about 87.3280869499.

The spelling of 665981 in words is "six hundred sixty-five thousand, nine hundred eighty-one".