81205561 has 2 divisors, whose sum is σ = 81205562. Its totient is φ = 81205560.

The previous prime is 81205559. The next prime is 81205577. The reversal of 81205561 is 16550218.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 51840000 + 29365561 = 7200^2 + 5419^2 .

It is a cyclic number.

It is not a de Polignac number, because 81205561 - 2¹ = 81205559 is a prime.

Together with 81205559, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (81205511) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40602780 + 40602781.

It is an arithmetic number, because the mean of its divisors is an integer number (40602781).

Almost surely, 2^81205561 is an apocalyptic number.

It is an amenable number.

81205561 is a deficient number, since it is larger than the sum of its proper divisors (1).

81205561 is an equidigital number, since it uses as much as digits as its factorization.

81205561 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2400, while the sum is 28.

The square root of 81205561 is about 9011.4128193086. The cubic root of 81205561 is about 433.0405745784.

Adding to 81205561 its reverse (16550218), we get a palindrome (97755779).

The spelling of 81205561 in words is "eighty-one million, two hundred five thousand, five hundred sixty-one".