81921757 has 4 divisors (see below), whose sum is σ = 81941436. Its totient is φ = 81902080.

The previous prime is 81921713. The next prime is 81921779. The reversal of 81921757 is 75712918.

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 2105401 + 79816356 = 1451^2 + 8934^2 .

It is a cyclic number.

It is not a de Polignac number, because 81921757 - 2¹⁹ = 81397469 is a prime.

It is a super-2 number, since 2×81921757² = 13422348539934098, 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 (81921797) 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 3 ways as a sum of consecutive naturals, for example, 868 + ... + 12829.

It is an arithmetic number, because the mean of its divisors is an integer number (20485359).

Almost surely, 2^81921757 is an apocalyptic number.

It is an amenable number.

81921757 is a deficient number, since it is larger than the sum of its proper divisors (19679).

81921757 is a wasteful number, since it uses less digits than its factorization.

81921757 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 19678.

The product of its digits is 35280, while the sum is 40.

The square root of 81921757 is about 9051.0638601216. The cubic root of 81921757 is about 434.3099236016.

The spelling of 81921757 in words is "eighty-one million, nine hundred twenty-one thousand, seven hundred fifty-seven".