661521561 has 32 divisors (see below), whose sum is σ = 977246208. Its totient is φ = 395915520.

The previous prime is 661521551. The next prime is 661521563. The reversal of 661521561 is 165125166.

It is not a de Polignac number, because 661521561 - 2¹¹ = 661519513 is a prime.

It is a super-2 number, since 2×661521561² = 875221551335753442, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (661521563) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 641116 + ... + 642146.

It is an arithmetic number, because the mean of its divisors is an integer number (30538944).

Almost surely, 2^661521561 is an apocalyptic number.

It is an amenable number.

661521561 is a deficient number, since it is larger than the sum of its proper divisors (315724647).

661521561 is a wasteful number, since it uses less digits than its factorization.

661521561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1621.

The product of its digits is 10800, while the sum is 33.

The square root of 661521561 is about 25720.0614501599. The cubic root of 661521561 is about 871.3273265401.

The spelling of 661521561 in words is "six hundred sixty-one million, five hundred twenty-one thousand, five hundred sixty-one".