21861 has 12 divisors (see below), whose sum is σ = 36192. Its totient is φ = 12456.

The previous prime is 21859. The next prime is 21863. The reversal of 21861 is 16812.

It is an interprime number because it is at equal distance from previous prime (21859) and next prime (21863).

It is not a de Polignac number, because 21861 - 2¹ = 21859 is a prime.

It is a super-3 number, since 3×21861³ = 31342333501143, which contains 333 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21863) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 111 + ... + 236.

It is an arithmetic number, because the mean of its divisors is an integer number (3016).

2²¹⁸⁶¹ is an apocalyptic number.

21861 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21861 is a deficient number, since it is larger than the sum of its proper divisors (14331).

21861 is a wasteful number, since it uses less digits than its factorization.

21861 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 360 (or 357 counting only the distinct ones).

The product of its digits is 96, while the sum is 18.

The square root of 21861 is about 147.8546583642. The cubic root of 21861 is about 27.9612559377.

It can be divided in two parts, 2186 and 1, that added together give a 7-th power (2187 = 3⁷).

The spelling of 21861 in words is "twenty-one thousand, eight hundred sixty-one".