1561555561 has 4 divisors (see below), whose sum is σ = 1599642324. Its totient is φ = 1523468800.

The previous prime is 1561555481. The next prime is 1561555571. The reversal of 1561555561 is 1655551651.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 177715561 + 1383840000 = 13331^2 + 37200^2 .

It is a cyclic number.

It is not a de Polignac number, because 1561555561 - 2²¹ = 1559458409 is a prime.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1561555571) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 19043320 + ... + 19043401.

It is an arithmetic number, because the mean of its divisors is an integer number (399910581).

Almost surely, 2^1561555561 is an apocalyptic number.

It is an amenable number.

1561555561 is a deficient number, since it is larger than the sum of its proper divisors (38086763).

1561555561 is an equidigital number, since it uses as much as digits as its factorization.

1561555561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 38086762.

The product of its digits is 112500, while the sum is 40.

The square root of 1561555561 is about 39516.5226329443. The cubic root of 1561555561 is about 1160.1633640810.

The spelling of 1561555561 in words is "one billion, five hundred sixty-one million, five hundred fifty-five thousand, five hundred sixty-one".