1561405666 has 4 divisors (see below), whose sum is σ = 2342108502. Its totient is φ = 780702832.

The previous prime is 1561405631. The next prime is 1561405673. The reversal of 1561405666 is 6665041651.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 6665041651 = 461 ⋅14457791.

It can be written as a sum of positive squares in only one way, i.e., 972130041 + 589275625 = 31179^2 + 24275^2 .

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 an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 390351415 + ... + 390351418.

Almost surely, 2^1561405666 is an apocalyptic number.

1561405666 is a deficient number, since it is larger than the sum of its proper divisors (780702836).

1561405666 is an equidigital number, since it uses as much as digits as its factorization.

1561405666 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 780702835.

The product of its (nonzero) digits is 129600, while the sum is 40.

The square root of 1561405666 is about 39514.6259757068. The cubic root of 1561405666 is about 1160.1262412153.

The spelling of 1561405666 in words is "one billion, five hundred sixty-one million, four hundred five thousand, six hundred sixty-six".