81309251 has 2 divisors, whose sum is σ = 81309252. Its totient is φ = 81309250.

The previous prime is 81309233. The next prime is 81309253. The reversal of 81309251 is 15290318.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 81309251 - 2¹⁸ = 81047107 is a prime.

It is a super-2 number, since 2×81309251² = 13222388596362002, which contains 22 as substring.

It is a Sophie Germain prime.

Together with 81309253, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (81309253) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40654625 + 40654626.

It is an arithmetic number, because the mean of its divisors is an integer number (40654626).

Almost surely, 2^81309251 is an apocalyptic number.

81309251 is a deficient number, since it is larger than the sum of its proper divisors (1).

81309251 is an equidigital number, since it uses as much as digits as its factorization.

81309251 is an odious number, because the sum of its binary digits is odd.

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

The square root of 81309251 is about 9017.1642438186. The cubic root of 81309251 is about 433.2248102336.

Adding to 81309251 its reverse (15290318), we get a palindrome (96599569).

The spelling of 81309251 in words is "eighty-one million, three hundred nine thousand, two hundred fifty-one".