81007781 has 4 divisors (see below), whose sum is σ = 81139692. Its totient is φ = 80875872.

The previous prime is 81007777. The next prime is 81007793. The reversal of 81007781 is 18770018.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 33535681 + 47472100 = 5791^2 + 6890^2 .

It is a cyclic number.

It is not a de Polignac number, because 81007781 - 2² = 81007777 is a prime.

It is a super-2 number, since 2×81007781² = 13124521165087922, 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 (81007481) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 65030 + ... + 66263.

It is an arithmetic number, because the mean of its divisors is an integer number (20284923).

Almost surely, 2^81007781 is an apocalyptic number.

It is an amenable number.

81007781 is a deficient number, since it is larger than the sum of its proper divisors (131911).

81007781 is a wasteful number, since it uses less digits than its factorization.

81007781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 131910.

The product of its (nonzero) digits is 3136, while the sum is 32.

The square root of 81007781 is about 9000.4322673969. The cubic root of 81007781 is about 432.6887251473.

Adding to 81007781 its reverse (18770018), we get a palindrome (99777799).

The spelling of 81007781 in words is "eighty-one million, seven thousand, seven hundred eighty-one".