81531391 has 2 divisors, whose sum is σ = 81531392. Its totient is φ = 81531390.

The previous prime is 81531379. The next prime is 81531403. The reversal of 81531391 is 19313518.

It is a balanced prime because it is at equal distance from previous prime (81531379) and next prime (81531403).

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81531391 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81531301) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40765695 + 40765696.

It is an arithmetic number, because the mean of its divisors is an integer number (40765696).

Almost surely, 2^81531391 is an apocalyptic number.

81531391 is a deficient number, since it is larger than the sum of its proper divisors (1).

81531391 is an equidigital number, since it uses as much as digits as its factorization.

81531391 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3240, while the sum is 31.

The square root of 81531391 is about 9029.4734619467. The cubic root of 81531391 is about 433.6189804347.

The spelling of 81531391 in words is "eighty-one million, five hundred thirty-one thousand, three hundred ninety-one".