81331561 has 2 divisors, whose sum is σ = 81331562. Its totient is φ = 81331560.

The previous prime is 81331489. The next prime is 81331589. The reversal of 81331561 is 16513318.

It is an a-pointer prime, because the next prime (81331589) can be obtained adding 81331561 to its sum of digits (28).

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 78216336 + 3115225 = 8844^2 + 1765^2 .

It is a cyclic number.

It is not a de Polignac number, because 81331561 - 2⁷ = 81331433 is a prime.

It is a super-2 number, since 2×81331561² = 13229645629393442, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (81338561) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40665780 + 40665781.

It is an arithmetic number, because the mean of its divisors is an integer number (40665781).

Almost surely, 2^81331561 is an apocalyptic number.

It is an amenable number.

81331561 is a deficient number, since it is larger than the sum of its proper divisors (1).

81331561 is an equidigital number, since it uses as much as digits as its factorization.

81331561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2160, while the sum is 28.

The square root of 81331561 is about 9018.4012441230. The cubic root of 81331561 is about 433.2644300043.

Adding to 81331561 its reverse (16513318), we get a palindrome (97844879).

The spelling of 81331561 in words is "eighty-one million, three hundred thirty-one thousand, five hundred sixty-one".