661551561 has 24 divisors (see below), whose sum is σ = 1048232640. Its totient is φ = 398714400.

The previous prime is 661551529. The next prime is 661551581. The reversal of 661551561 is 165155166.

It is not a de Polignac number, because 661551561 - 2⁵ = 661551529 is a prime.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (661551521) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 541 + ... + 36378.

It is an arithmetic number, because the mean of its divisors is an integer number (43676360).

Almost surely, 2^661551561 is an apocalyptic number.

It is an amenable number.

661551561 is a deficient number, since it is larger than the sum of its proper divisors (386681079).

661551561 is a wasteful number, since it uses less digits than its factorization.

661551561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 37117 (or 37114 counting only the distinct ones).

The product of its digits is 27000, while the sum is 36.

The square root of 661551561 is about 25720.6446458871. The cubic root of 661551561 is about 871.3404979045.

The spelling of 661551561 in words is "six hundred sixty-one million, five hundred fifty-one thousand, five hundred sixty-one".