81533509 has 2 divisors, whose sum is σ = 81533510. Its totient is φ = 81533508.

The previous prime is 81533449. The next prime is 81533521. The reversal of 81533509 is 90533518.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 64352484 + 17181025 = 8022^2 + 4145^2 .

It is a cyclic number.

It is not a de Polignac number, because 81533509 - 2⁷ = 81533381 is a prime.

It is equal to p_4753807 and since 81533509 and 4753807 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81533539) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40766754 + 40766755.

It is an arithmetic number, because the mean of its divisors is an integer number (40766755).

Almost surely, 2^81533509 is an apocalyptic number.

It is an amenable number.

81533509 is a deficient number, since it is larger than the sum of its proper divisors (1).

81533509 is an equidigital number, since it uses as much as digits as its factorization.

81533509 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 16200, while the sum is 34.

The square root of 81533509 is about 9029.5907437713. The cubic root of 81533509 is about 433.6227352136.

Subtracting 81533509 from its reverse (90533518), we obtain a palindrome (9000009).

The spelling of 81533509 in words is "eighty-one million, five hundred thirty-three thousand, five hundred nine".