661691581 has 4 divisors (see below), whose sum is σ = 662675452. Its totient is φ = 660707712.

The previous prime is 661691573. The next prime is 661691599. The reversal of 661691581 is 185196166.

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 248220025 + 413471556 = 15755^2 + 20334^2 .

It is a cyclic number.

It is not a de Polignac number, because 661691581 - 2³ = 661691573 is a prime.

It is a super-2 number, since 2×661691581² = 875671496732559122, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (661691081) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 490926 + ... + 492271.

It is an arithmetic number, because the mean of its divisors is an integer number (165668863).

Almost surely, 2^661691581 is an apocalyptic number.

It is an amenable number.

661691581 is a deficient number, since it is larger than the sum of its proper divisors (983871).

661691581 is an equidigital number, since it uses as much as digits as its factorization.

661691581 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 983870.

The product of its digits is 77760, while the sum is 43.

The square root of 661691581 is about 25723.3664398733. The cubic root of 661691581 is about 871.4019677869.

The spelling of 661691581 in words is "six hundred sixty-one million, six hundred ninety-one thousand, five hundred eighty-one".