81723781 has 4 divisors (see below), whose sum is σ = 84360064. Its totient is φ = 79087500.

The previous prime is 81723773. The next prime is 81723797. The reversal of 81723781 is 18732718.

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 81723781 - 2³ = 81723773 is a prime.

It is a super-2 number, since 2×81723781² = 13357552761871922, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 81723781.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81723881) 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, 1318095 + ... + 1318156.

It is an arithmetic number, because the mean of its divisors is an integer number (21090016).

Almost surely, 2^81723781 is an apocalyptic number.

It is an amenable number.

81723781 is a deficient number, since it is larger than the sum of its proper divisors (2636283).

81723781 is a wasteful number, since it uses less digits than its factorization.

81723781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2636282.

The product of its digits is 18816, while the sum is 37.

The square root of 81723781 is about 9040.1206297261. The cubic root of 81723781 is about 433.9597834141.

The spelling of 81723781 in words is "eighty-one million, seven hundred twenty-three thousand, seven hundred eighty-one".