81390021 has 4 divisors (see below), whose sum is σ = 108520032. Its totient is φ = 54260012.

The previous prime is 81390019. The next prime is 81390061. The reversal of 81390021 is 12009318.

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.

It is a cyclic number.

It is not a de Polignac number, because 81390021 - 2¹ = 81390019 is a prime.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 81389973 and 81390000.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81390061) 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, 13565001 + ... + 13565006.

It is an arithmetic number, because the mean of its divisors is an integer number (27130008).

Almost surely, 2^81390021 is an apocalyptic number.

It is an amenable number.

81390021 is a deficient number, since it is larger than the sum of its proper divisors (27130011).

81390021 is a wasteful number, since it uses less digits than its factorization.

81390021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 27130010.

The product of its (nonzero) digits is 432, while the sum is 24.

The square root of 81390021 is about 9021.6418128853. The cubic root of 81390021 is about 433.3682133002.

Adding to 81390021 its reverse (12009318), we get a palindrome (93399339).

The spelling of 81390021 in words is "eighty-one million, three hundred ninety thousand, twenty-one".