81821221 has 8 divisors (see below), whose sum is σ = 86725080. Its totient is φ = 76927488.

The previous prime is 81821213. The next prime is 81821227. The reversal of 81821221 is 12212818.

It can be written as a sum of positive squares in 4 ways, for example, as 42068196 + 39753025 = 6486^2 + 6305^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 81821221 - 2³ = 81821213 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 (81821227) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 19825 + ... + 23593.

It is an arithmetic number, because the mean of its divisors is an integer number (10840635).

Almost surely, 2^81821221 is an apocalyptic number.

It is an amenable number.

81821221 is a deficient number, since it is larger than the sum of its proper divisors (4903859).

81821221 is a wasteful number, since it uses less digits than its factorization.

81821221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5063.

The product of its digits is 512, while the sum is 25.

The square root of 81821221 is about 9045.5083328689. The cubic root of 81821221 is about 434.1321862989.

The spelling of 81821221 in words is "eighty-one million, eight hundred twenty-one thousand, two hundred twenty-one".