815561561 has 2 divisors, whose sum is σ = 815561562. Its totient is φ = 815561560.

The previous prime is 815561503. The next prime is 815561563. The reversal of 815561561 is 165165518.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 796368400 + 19193161 = 28220^2 + 4381^2 .

It is a cyclic number.

It is not a de Polignac number, because 815561561 - 2¹⁴ = 815545177 is a prime.

Together with 815561563, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (815561563) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 407780780 + 407780781.

It is an arithmetic number, because the mean of its divisors is an integer number (407780781).

Almost surely, 2^815561561 is an apocalyptic number.

It is an amenable number.

815561561 is a deficient number, since it is larger than the sum of its proper divisors (1).

815561561 is an equidigital number, since it uses as much as digits as its factorization.

815561561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 36000, while the sum is 38.

The square root of 815561561 is about 28558.0384655529. The cubic root of 815561561 is about 934.2983520323.

The spelling of 815561561 in words is "eight hundred fifteen million, five hundred sixty-one thousand, five hundred sixty-one".