81218561 has 4 divisors (see below), whose sum is σ = 81335124. Its totient is φ = 81102000.

The previous prime is 81218513. The next prime is 81218567. The reversal of 81218561 is 16581218.

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 28569025 + 52649536 = 5345^2 + 7256^2 .

It is a cyclic number.

It is not a de Polignac number, because 81218561 - 2¹⁸ = 80956417 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81218567) 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, 57230 + ... + 58631.

It is an arithmetic number, because the mean of its divisors is an integer number (20333781).

Almost surely, 2^81218561 is an apocalyptic number.

It is an amenable number.

81218561 is a deficient number, since it is larger than the sum of its proper divisors (116563).

81218561 is a wasteful number, since it uses less digits than its factorization.

81218561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 116562.

The product of its digits is 3840, while the sum is 32.

The square root of 81218561 is about 9012.1340979815. The cubic root of 81218561 is about 433.0636814815.

Adding to 81218561 its reverse (16581218), we get a palindrome (97799779).

The spelling of 81218561 in words is "eighty-one million, two hundred eighteen thousand, five hundred sixty-one".