1661563021 has 4 divisors (see below), whose sum is σ = 1664634844. Its totient is φ = 1658491200.

The previous prime is 1661563009. The next prime is 1661563063. The reversal of 1661563021 is 1203651661.

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 1186802500 + 474760521 = 34450^2 + 21789^2 .

It is a cyclic number.

It is not a de Polignac number, because 1661563021 - 2¹⁹ = 1661038733 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1661563081) 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, 1535100 + ... + 1536181.

It is an arithmetic number, because the mean of its divisors is an integer number (416158711).

Almost surely, 2^1661563021 is an apocalyptic number.

It is an amenable number.

1661563021 is a deficient number, since it is larger than the sum of its proper divisors (3071823).

1661563021 is an equidigital number, since it uses as much as digits as its factorization.

1661563021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3071822.

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

The square root of 1661563021 is about 40762.2744826635. The cubic root of 1661563021 is about 1184.4196558894.

The spelling of 1661563021 in words is "one billion, six hundred sixty-one million, five hundred sixty-three thousand, twenty-one".