81909781 has 8 divisors (see below), whose sum is σ = 85164800. Its totient is φ = 78693120.

The previous prime is 81909761. The next prime is 81909787. The reversal of 81909781 is 18790918.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81909781 is a prime.

It is a super-2 number, since 2×81909781² = 13418424446935922, 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 (81909787) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 5196 + ... + 13813.

It is an arithmetic number, because the mean of its divisors is an integer number (10645600).

Almost surely, 2^81909781 is an apocalyptic number.

It is an amenable number.

81909781 is a deficient number, since it is larger than the sum of its proper divisors (3255019).

81909781 is a wasteful number, since it uses less digits than its factorization.

81909781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 19179.

The product of its (nonzero) digits is 36288, while the sum is 43.

The square root of 81909781 is about 9050.4022562536. The cubic root of 81909781 is about 434.2887588980.

It can be divided in two parts, 8190 and 9781, that added together give a palindrome (17971).

The spelling of 81909781 in words is "eighty-one million, nine hundred nine thousand, seven hundred eighty-one".