81685991 has 4 divisors (see below), whose sum is σ = 82273800. Its totient is φ = 81098184.

The previous prime is 81685963. The next prime is 81685999. The reversal of 81685991 is 19958618.

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

It is a cyclic number.

It is not a de Polignac number, because 81685991 - 2¹⁴ = 81669607 is a prime.

It is a super-2 number, since 2×81685991² = 13345202251304162, 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 (81685999) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 293696 + ... + 293973.

It is an arithmetic number, because the mean of its divisors is an integer number (20568450).

Almost surely, 2^81685991 is an apocalyptic number.

81685991 is a deficient number, since it is larger than the sum of its proper divisors (587809).

81685991 is a wasteful number, since it uses less digits than its factorization.

81685991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 587808.

The product of its digits is 155520, while the sum is 47.

The square root of 81685991 is about 9038.0302610691. The cubic root of 81685991 is about 433.8928837985.

The spelling of 81685991 in words is "eighty-one million, six hundred eighty-five thousand, nine hundred ninety-one".