81644021 has 4 divisors (see below), whose sum is σ = 81664044. Its totient is φ = 81624000.

The previous prime is 81644011. The next prime is 81644033. The reversal of 81644021 is 12044618.

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

It can be written as a sum of positive squares in 2 ways, for example, as 29008996 + 52635025 = 5386^2 + 7255^2 .

It is a cyclic number.

It is not a de Polignac number, because 81644021 - 2¹⁰ = 81642997 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81644011) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1460 + ... + 12861.

It is an arithmetic number, because the mean of its divisors is an integer number (20416011).

Almost surely, 2^81644021 is an apocalyptic number.

It is an amenable number.

81644021 is a deficient number, since it is larger than the sum of its proper divisors (20023).

81644021 is a wasteful number, since it uses less digits than its factorization.

81644021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 20022.

The product of its (nonzero) digits is 1536, while the sum is 26.

The square root of 81644021 is about 9035.7081072819. The cubic root of 81644021 is about 433.8185601442.

Adding to 81644021 its reverse (12044618), we get a palindrome (93688639).

The spelling of 81644021 in words is "eighty-one million, six hundred forty-four thousand, twenty-one".