213781 has 4 divisors (see below), whose sum is σ = 216864. Its totient is φ = 210700.

The previous prime is 213751. The next prime is 213791. The reversal of 213781 is 187312.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 213781 - 2¹³ = 205589 is a prime.

It is a super-2 number, since 2×213781² = 91404631922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (213721) 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, 1435 + ... + 1576.

It is an arithmetic number, because the mean of its divisors is an integer number (54216).

2²¹³⁷⁸¹ is an apocalyptic number.

It is an amenable number.

213781 is a deficient number, since it is larger than the sum of its proper divisors (3083).

213781 is an equidigital number, since it uses as much as digits as its factorization.

213781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3082.

The product of its digits is 336, while the sum is 22.

The square root of 213781 is about 462.3645747676. The cubic root of 213781 is about 59.7938294099.

It can be divided in two parts, 21 and 3781, that multiplied together give a triangular number (79401 = T₃₉₈).

The spelling of 213781 in words is "two hundred thirteen thousand, seven hundred eighty-one".