81638209 has 4 divisors (see below), whose sum is σ = 83536816. Its totient is φ = 79739604.

The previous prime is 81638203. The next prime is 81638213. The reversal of 81638209 is 90283618.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 90283618 = 2 ⋅45141809.

It is a cyclic number.

It is not a de Polignac number, because 81638209 - 2⁵ = 81638177 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81638203) 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, 949239 + ... + 949324.

It is an arithmetic number, because the mean of its divisors is an integer number (20884204).

Almost surely, 2^81638209 is an apocalyptic number.

It is an amenable number.

81638209 is a deficient number, since it is larger than the sum of its proper divisors (1898607).

81638209 is a wasteful number, since it uses less digits than its factorization.

81638209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1898606.

The product of its (nonzero) digits is 20736, while the sum is 37.

The square root of 81638209 is about 9035.3864886899. The cubic root of 81638209 is about 433.8082658068.

The spelling of 81638209 in words is "eighty-one million, six hundred thirty-eight thousand, two hundred nine".