21761 has 4 divisors (see below), whose sum is σ = 22272. Its totient is φ = 21252.

The previous prime is 21757. The next prime is 21767. The reversal of 21761 is 16712.

It is a happy number.

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 21761 - 2² = 21757 is a prime.

It is a super-2 number, since 2×21761² = 947082242, which contains 22 as substring.

It is a Duffinian number.

It is a plaindrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (21767) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 185 + ... + 278.

It is an arithmetic number, because the mean of its divisors is an integer number (5568).

It is a Proth number, since it is equal to 85 ⋅ 2⁸ + 1 and 85 < 2⁸.

2²¹⁷⁶¹ is an apocalyptic number.

It is an amenable number.

21761 is a deficient number, since it is larger than the sum of its proper divisors (511).

21761 is an equidigital number, since it uses as much as digits as its factorization.

21761 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 510.

The product of its digits is 84, while the sum is 17.

The square root of 21761 is about 147.5161008161. The cubic root of 21761 is about 27.9185558485.

The spelling of 21761 in words is "twenty-one thousand, seven hundred sixty-one".