81555517 has 2 divisors, whose sum is σ = 81555518. Its totient is φ = 81555516.

The previous prime is 81555497. The next prime is 81555533. The reversal of 81555517 is 71555518.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 57062916 + 24492601 = 7554^2 + 4949^2 .

It is a cyclic number.

It is not a de Polignac number, because 81555517 - 2²³ = 73166909 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81555547) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40777758 + 40777759.

It is an arithmetic number, because the mean of its divisors is an integer number (40777759).

Almost surely, 2^81555517 is an apocalyptic number.

It is an amenable number.

81555517 is a deficient number, since it is larger than the sum of its proper divisors (1).

81555517 is an equidigital number, since it uses as much as digits as its factorization.

81555517 is an evil number, because the sum of its binary digits is even.

The product of its digits is 35000, while the sum is 37.

The square root of 81555517 is about 9030.8093214285. The cubic root of 81555517 is about 433.6617470292.

Subtracting from 81555517 its reverse (71555518), we obtain a palindrome (9999999).

The spelling of 81555517 in words is "eighty-one million, five hundred fifty-five thousand, five hundred seventeen".