21668 has 6 divisors (see below), whose sum is σ = 37926. Its totient is φ = 10832.

The previous prime is 21661. The next prime is 21673. The reversal of 21668 is 86612.

It can be written as a sum of positive squares in only one way, i.e., 13924 + 7744 = 118^2 + 88^2 .

It is not an unprimeable number, because it can be changed into a prime (21661) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 2705 + ... + 2712.

It is an arithmetic number, because the mean of its divisors is an integer number (6321).

2²¹⁶⁶⁸ is an apocalyptic number.

It is an amenable number.

21668 is a deficient number, since it is larger than the sum of its proper divisors (16258).

21668 is a wasteful number, since it uses less digits than its factorization.

21668 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5421 (or 5419 counting only the distinct ones).

The product of its digits is 576, while the sum is 23.

The square root of 21668 is about 147.2005434773. The cubic root of 21668 is about 27.8787272058.

It can be divided in two parts, 21 and 668, that multiplied together give a triangular number (14028 = T₁₆₇).

The spelling of 21668 in words is "twenty-one thousand, six hundred sixty-eight".