261821 has 8 divisors (see below), whose sum is σ = 302784. Its totient is φ = 221760.

The previous prime is 261799. The next prime is 261823. The reversal of 261821 is 128162.

261821 = T₉₀ + T₉₁ + ... + T₁₃₁.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 261821 - 2⁶ = 261757 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 261793 and 261802.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (261823) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 626 + ... + 956.

It is an arithmetic number, because the mean of its divisors is an integer number (37848).

2²⁶¹⁸²¹ is an apocalyptic number.

It is an amenable number.

261821 is a deficient number, since it is larger than the sum of its proper divisors (40963).

261821 is a wasteful number, since it uses less digits than its factorization.

261821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 451.

The product of its digits is 192, while the sum is 20.

The square root of 261821 is about 511.6844730886. The cubic root of 261821 is about 63.9737033893.

Adding to 261821 its reverse (128162), we get a palindrome (389983).

The spelling of 261821 in words is "two hundred sixty-one thousand, eight hundred twenty-one".