81994781 has 4 divisors (see below), whose sum is σ = 89448864. Its totient is φ = 74540700.

The previous prime is 81994777. The next prime is 81994789. The reversal of 81994781 is 18749918.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 81994781 - 2² = 81994777 is a prime.

It is a super-2 number, since 2×81994781² = 13446288222475922, 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 (81994789) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3727025 + ... + 3727046.

It is an arithmetic number, because the mean of its divisors is an integer number (22362216).

Almost surely, 2^81994781 is an apocalyptic number.

It is an amenable number.

81994781 is a deficient number, since it is larger than the sum of its proper divisors (7454083).

81994781 is a wasteful number, since it uses less digits than its factorization.

81994781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7454082.

The product of its digits is 145152, while the sum is 47.

The square root of 81994781 is about 9055.0969624847. The cubic root of 81994781 is about 434.4389313696.

It can be divided in two parts, 819947 and 81, that added together give a palindrome (820028).

The spelling of 81994781 in words is "eighty-one million, nine hundred ninety-four thousand, seven hundred eighty-one".