81705691 has 2 divisors, whose sum is σ = 81705692.
Its totient is φ = 81705690.
The previous prime is 81705653. The next prime is 81705703. The reversal of 81705691 is 19650718.
It is a strong prime.
It is a cyclic number.
It is not a de Polignac number, because 81705691 - 27 = 81705563 is a prime.
It is a self number, because there is not a number n which added to its sum of digits gives 81705691.
It is not a weakly prime, because it can be changed into another prime (81705191) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40852845 + 40852846.
It is an arithmetic number, because the mean of its divisors is an integer number (40852846).
Almost surely, 281705691 is an apocalyptic number.
81705691 is a deficient number, since it is larger than the sum of its proper divisors (1).
81705691 is an equidigital number, since it uses as much as digits as its factorization.
81705691 is an evil number, because the sum of its binary digits is even.
The product of its (nonzero) digits is 15120, while the sum is 37.
The square root of 81705691 is about 9039.1200346051.
The cubic root of 81705691 is about 433.9277612712.
It can be divided in two parts, 8170 and 5691, that added together give a triangular number (13861 = T166).
The spelling of 81705691 in words is "eighty-one million, seven hundred five thousand, six hundred ninety-one".