81581391 has 16 divisors (see below), whose sum is σ = 126117120. Its totient is φ = 52022520.

The previous prime is 81581389. The next prime is 81581393. The reversal of 81581391 is 19318518.

It is an interprime number because it is at equal distance from previous prime (81581389) and next prime (81581393).

It is not a de Polignac number, because 81581391 - 2¹ = 81581389 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81581393) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 65065 + ... + 66306.

It is an arithmetic number, because the mean of its divisors is an integer number (7882320).

Almost surely, 2^81581391 is an apocalyptic number.

81581391 is a deficient number, since it is larger than the sum of its proper divisors (44535729).

81581391 is a wasteful number, since it uses less digits than its factorization.

81581391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 131403 (or 131397 counting only the distinct ones).

The product of its digits is 8640, while the sum is 36.

The square root of 81581391 is about 9032.2417483148. The cubic root of 81581391 is about 433.7076028178.

The spelling of 81581391 in words is "eighty-one million, five hundred eighty-one thousand, three hundred ninety-one".