261991 has 4 divisors (see below), whose sum is σ = 275800. Its totient is φ = 248184.

The previous prime is 261983. The next prime is 262007. The reversal of 261991 is 199162.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199162 = 2 ⋅99581.

It is a cyclic number.

It is not a de Polignac number, because 261991 - 2³ = 261983 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 (261971) 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, 6876 + ... + 6913.

It is an arithmetic number, because the mean of its divisors is an integer number (68950).

2²⁶¹⁹⁹¹ is an apocalyptic number.

261991 is a deficient number, since it is larger than the sum of its proper divisors (13809).

261991 is a wasteful number, since it uses less digits than its factorization.

261991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 13808.

The product of its digits is 972, while the sum is 28.

The square root of 261991 is about 511.8505641298. The cubic root of 261991 is about 63.9875464050.

The spelling of 261991 in words is "two hundred sixty-one thousand, nine hundred ninety-one".