261391 has 4 divisors (see below), whose sum is σ = 281512. Its totient is φ = 241272.

The previous prime is 261389. The next prime is 261407. The reversal of 261391 is 193162.

261391 = T₁₉₄ + T₁₉₅ + ... + T₂₀₆.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 193162 = 2 ⋅96581.

It is a cyclic number.

It is not a de Polignac number, because 261391 - 2¹ = 261389 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 (261301) 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, 10041 + ... + 10066.

It is an arithmetic number, because the mean of its divisors is an integer number (70378).

2²⁶¹³⁹¹ is an apocalyptic number.

261391 is a deficient number, since it is larger than the sum of its proper divisors (20121).

261391 is a wasteful number, since it uses less digits than its factorization.

261391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 20120.

The product of its digits is 324, while the sum is 22.

The square root of 261391 is about 511.2641196094. The cubic root of 261391 is about 63.9386619351.

It can be divided in two parts, 2613 and 91, that added together give a square (2704 = 52²).

The spelling of 261391 in words is "two hundred sixty-one thousand, three hundred ninety-one".