81890721 has 24 divisors (see below), whose sum is σ = 129617280. Its totient is φ = 49408800.

The previous prime is 81890707. The next prime is 81890723. The reversal of 81890721 is 12709818.

It is not a de Polignac number, because 81890721 - 2¹³ = 81882529 is a prime.

It is a Smith number, since the sum of its digits (36) coincides with the sum of the digits of its prime factors.

It is an alternating number because its digits alternate between even and odd.

It is not an unprimeable number, because it can be changed into a prime (81890723) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 21931 + ... + 25391.

It is an arithmetic number, because the mean of its divisors is an integer number (5400720).

Almost surely, 2^81890721 is an apocalyptic number.

It is an amenable number.

81890721 is a deficient number, since it is larger than the sum of its proper divisors (47726559).

81890721 is a wasteful number, since it uses less digits than its factorization.

81890721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3717 (or 3714 counting only the distinct ones).

The product of its (nonzero) digits is 8064, while the sum is 36.

The square root of 81890721 is about 9049.3492031195. The cubic root of 81890721 is about 434.2550706710.

The spelling of 81890721 in words is "eighty-one million, eight hundred ninety thousand, seven hundred twenty-one".