261781 has 6 divisors (see below), whose sum is σ = 283650. Its totient is φ = 241488.

The previous prime is 261773. The next prime is 261787. The reversal of 261781 is 187162.

It can be written as a sum of positive squares in 3 ways, for example, as 1681 + 260100 = 41^2 + 510^2 .

It is not a de Polignac number, because 261781 - 2³ = 261773 is a prime.

It is a super-2 number, since 2×261781² = 137058583922, which contains 22 as substring.

It is a Duffinian number.

261781 is a lucky number.

It is a nialpdrome in base 4 and base 13.

It is a zygodrome in base 4.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (261787) 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 5 ways as a sum of consecutive naturals, for example, 606 + ... + 943.

It is an arithmetic number, because the mean of its divisors is an integer number (47275).

2²⁶¹⁷⁸¹ is an apocalyptic number.

It is an amenable number.

261781 is a deficient number, since it is larger than the sum of its proper divisors (21869).

261781 is a wasteful number, since it uses less digits than its factorization.

261781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1575 (or 1562 counting only the distinct ones).

The product of its digits is 672, while the sum is 25.

The square root of 261781 is about 511.6453850080. The cubic root of 261781 is about 63.9704453384.

It can be divided in two parts, 261 and 781, that multiplied together give a triangular number (203841 = T₆₃₈).

The spelling of 261781 in words is "two hundred sixty-one thousand, seven hundred eighty-one".