261661 has 4 divisors (see below), whose sum is σ = 266652. Its totient is φ = 256672.

The previous prime is 261643. The next prime is 261673. The reversal of 261661 is 166162.

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 152100 + 109561 = 390^2 + 331^2 .

It is a cyclic number.

It is not a de Polignac number, because 261661 - 2¹³ = 253469 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (261601) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2416 + ... + 2521.

It is an arithmetic number, because the mean of its divisors is an integer number (66663).

2²⁶¹⁶⁶¹ is an apocalyptic number.

It is an amenable number.

261661 is a deficient number, since it is larger than the sum of its proper divisors (4991).

261661 is an equidigital number, since it uses as much as digits as its factorization.

261661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4990.

The product of its digits is 432, while the sum is 22.

The square root of 261661 is about 511.5281028448. The cubic root of 261661 is about 63.9606691938.

The spelling of 261661 in words is "two hundred sixty-one thousand, six hundred sixty-one".