81309521 has 4 divisors (see below), whose sum is σ = 86092452. Its totient is φ = 76526592.

The previous prime is 81309511. The next prime is 81309563. The reversal of 81309521 is 12590318.

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 73960000 + 7349521 = 8600^2 + 2711^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81309521 is a prime.

It is a super-2 number, since 2×81309521² = 13222476410498882, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81309511) 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, 2391440 + ... + 2391473.

It is an arithmetic number, because the mean of its divisors is an integer number (21523113).

Almost surely, 2^81309521 is an apocalyptic number.

It is an amenable number.

81309521 is a deficient number, since it is larger than the sum of its proper divisors (4782931).

81309521 is a wasteful number, since it uses less digits than its factorization.

81309521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4782930.

The product of its (nonzero) digits is 2160, while the sum is 29.

The square root of 81309521 is about 9017.1792152535. The cubic root of 81309521 is about 433.2252897631.

Adding to 81309521 its reverse (12590318), we get a palindrome (93899839).

The spelling of 81309521 in words is "eighty-one million, three hundred nine thousand, five hundred twenty-one".