81987809 has 2 divisors, whose sum is σ = 81987810. Its totient is φ = 81987808.

The previous prime is 81987803. The next prime is 81987811. The reversal of 81987809 is 90878918.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 81504784 + 483025 = 9028^2 + 695^2 .

It is a cyclic number.

It is not a de Polignac number, because 81987809 - 2¹⁶ = 81922273 is a prime.

It is a Sophie Germain prime.

Together with 81987811, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (81987803) 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, 40993904 + 40993905.

It is an arithmetic number, because the mean of its divisors is an integer number (40993905).

Almost surely, 2^81987809 is an apocalyptic number.

It is an amenable number.

81987809 is a deficient number, since it is larger than the sum of its proper divisors (1).

81987809 is an equidigital number, since it uses as much as digits as its factorization.

81987809 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 290304, while the sum is 50.

The square root of 81987809 is about 9054.7119777495. The cubic root of 81987809 is about 434.4266176018.

The spelling of 81987809 in words is "eighty-one million, nine hundred eighty-seven thousand, eight hundred nine".