661566561 has 16 divisors (see below), whose sum is σ = 889929984. Its totient is φ = 437134464.

The previous prime is 661566559. The next prime is 661566571. The reversal of 661566561 is 165665166.

It is not a de Polignac number, because 661566561 - 2¹ = 661566559 is a prime.

It is not an unprimeable number, because it can be changed into a prime (661566551) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 497880 + ... + 499206.

It is an arithmetic number, because the mean of its divisors is an integer number (55620624).

Almost surely, 2^661566561 is an apocalyptic number.

It is an amenable number.

661566561 is a deficient number, since it is larger than the sum of its proper divisors (228363423).

661566561 is a wasteful number, since it uses less digits than its factorization.

661566561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2680.

The product of its digits is 194400, while the sum is 42.

The square root of 661566561 is about 25720.9362387919. The cubic root of 661566561 is about 871.3470834374.

The spelling of 661566561 in words is "six hundred sixty-one million, five hundred sixty-six thousand, five hundred sixty-one".