261691 has 4 divisors (see below), whose sum is σ = 264384. Its totient is φ = 259000.

The previous prime is 261673. The next prime is 261697. The reversal of 261691 is 196162.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 196162 = 2 ⋅98081.

It is a cyclic number.

It is not a de Polignac number, because 261691 - 2⁷ = 261563 is a prime.

It is a Duffinian number.

261691 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (261697) 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, 1195 + ... + 1396.

It is an arithmetic number, because the mean of its divisors is an integer number (66096).

2²⁶¹⁶⁹¹ is an apocalyptic number.

261691 is a deficient number, since it is larger than the sum of its proper divisors (2693).

261691 is a wasteful number, since it uses less digits than its factorization.

261691 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2692.

The product of its digits is 648, while the sum is 25.

The square root of 261691 is about 511.5574259064. The cubic root of 261691 is about 63.9631135101.

The spelling of 261691 in words is "two hundred sixty-one thousand, six hundred ninety-one".