1820131561 has 4 divisors (see below), whose sum is σ = 1820218324. Its totient is φ = 1820044800.

The previous prime is 1820131559. The next prime is 1820131601. The reversal of 1820131561 is 1651310281.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 1651310281 = 36599 ⋅45119.

It can be written as a sum of positive squares in 2 ways, for example, as 1188111961 + 632019600 = 34469^2 + 25140^2 .

It is a cyclic number.

It is not a de Polignac number, because 1820131561 - 2¹ = 1820131559 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1820131531) 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, 9901 + ... + 61141.

It is an arithmetic number, because the mean of its divisors is an integer number (455054581).

Almost surely, 2^1820131561 is an apocalyptic number.

It is an amenable number.

1820131561 is a deficient number, since it is larger than the sum of its proper divisors (86763).

1820131561 is an equidigital number, since it uses as much as digits as its factorization.

1820131561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 86762.

The product of its (nonzero) digits is 1440, while the sum is 28.

The square root of 1820131561 is about 42662.9999062419. Note that the first 4 decimals coincide. The cubic root of 1820131561 is about 1220.9585678401.

The spelling of 1820131561 in words is "one billion, eight hundred twenty million, one hundred thirty-one thousand, five hundred sixty-one".