1561166021 has 4 divisors (see below), whose sum is σ = 1561830732. Its totient is φ = 1560501312.

The previous prime is 1561166017. The next prime is 1561166023. The reversal of 1561166021 is 1206611651.

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 222337921 + 1338828100 = 14911^2 + 36590^2 .

It is a cyclic number.

It is not a de Polignac number, because 1561166021 - 2² = 1561166017 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1561166023) by changing a digit.

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 in 3 ways as a sum of consecutive naturals, for example, 328820 + ... + 333533.

It is an arithmetic number, because the mean of its divisors is an integer number (390457683).

Almost surely, 2^1561166021 is an apocalyptic number.

It is an amenable number.

1561166021 is a deficient number, since it is larger than the sum of its proper divisors (664711).

1561166021 is an equidigital number, since it uses as much as digits as its factorization.

1561166021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 664710.

The product of its (nonzero) digits is 2160, while the sum is 29.

The square root of 1561166021 is about 39511.5935011485. The cubic root of 1561166021 is about 1160.0668860065.

Adding to 1561166021 its reverse (1206611651), we get a palindrome (2767777672).

The spelling of 1561166021 in words is "one billion, five hundred sixty-one million, one hundred sixty-six thousand, twenty-one".