1561666 has 4 divisors (see below), whose sum is σ = 2342502. Its totient is φ = 780832.

The previous prime is 1561657. The next prime is 1561673. The reversal of 1561666 is 6661651.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 6661651 = 23 ⋅289637.

It can be written as a sum of positive squares in only one way, i.e., 1030225 + 531441 = 1015^2 + 729^2 .

It is a Smith number, since the sum of its digits (31) 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 polite number, since it can be written as a sum of consecutive naturals, namely, 390415 + ... + 390418.

2^1561666 is an apocalyptic number.

1561666 is a deficient number, since it is larger than the sum of its proper divisors (780836).

1561666 is an equidigital number, since it uses as much as digits as its factorization.

1561666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 780835.

The product of its digits is 6480, while the sum is 31.

The square root of 1561666 is about 1249.6663554725. Note that the first 3 decimals coincide. The cubic root of 1561666 is about 116.0190713788.

The spelling of 1561666 in words is "one million, five hundred sixty-one thousand, six hundred sixty-six".