81721399 has 4 divisors (see below), whose sum is σ = 81754480. Its totient is φ = 81688320.

The previous prime is 81721379. The next prime is 81721429. The reversal of 81721399 is 99312718.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 99312718 = 2 ⋅49656359.

It is a cyclic number.

It is not a de Polignac number, because 81721399 - 2¹⁹ = 81197111 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 (81721379) 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, 12507 + ... + 17884.

It is an arithmetic number, because the mean of its divisors is an integer number (20438620).

Almost surely, 2^81721399 is an apocalyptic number.

81721399 is a deficient number, since it is larger than the sum of its proper divisors (33081).

81721399 is a wasteful number, since it uses less digits than its factorization.

81721399 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 33080.

The product of its digits is 27216, while the sum is 40.

The square root of 81721399 is about 9039.9888827365. The cubic root of 81721399 is about 433.9555671699.

The spelling of 81721399 in words is "eighty-one million, seven hundred twenty-one thousand, three hundred ninety-nine".