661561 has 4 divisors (see below), whose sum is σ = 696400. Its totient is φ = 626724.

The previous prime is 661553. The next prime is 661603. The reversal of 661561 is 165166.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 661561 - 2³ = 661553 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (661541) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 17391 + ... + 17428.

It is an arithmetic number, because the mean of its divisors is an integer number (174100).

2⁶⁶¹⁵⁶¹ is an apocalyptic number.

It is an amenable number.

661561 is a deficient number, since it is larger than the sum of its proper divisors (34839).

661561 is a wasteful number, since it uses less digits than its factorization.

661561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 34838.

The product of its digits is 1080, while the sum is 25.

The square root of 661561 is about 813.3640021540. The cubic root of 661561 is about 87.1344641972.

The spelling of 661561 in words is "six hundred sixty-one thousand, five hundred sixty-one".