81666649 has 2 divisors, whose sum is σ = 81666650. Its totient is φ = 81666648.

The previous prime is 81666647. The next prime is 81666653. The reversal of 81666649 is 94666618.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 75255625 + 6411024 = 8675^2 + 2532^2 .

It is a cyclic number.

It is not a de Polignac number, because 81666649 - 2¹ = 81666647 is a prime.

Together with 81666647, it forms a pair of twin primes.

It is a junction number, because it is equal to n+sod(n) for n = 81666599 and 81666608.

It is not a weakly prime, because it can be changed into another prime (81666647) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40833324 + 40833325.

It is an arithmetic number, because the mean of its divisors is an integer number (40833325).

Almost surely, 2^81666649 is an apocalyptic number.

It is an amenable number.

81666649 is a deficient number, since it is larger than the sum of its proper divisors (1).

81666649 is an equidigital number, since it uses as much as digits as its factorization.

81666649 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 373248, while the sum is 46.

The square root of 81666649 is about 9036.9601636834. The cubic root of 81666649 is about 433.8586346837.

The spelling of 81666649 in words is "eighty-one million, six hundred sixty-six thousand, six hundred forty-nine".