21681668 has 6 divisors (see below), whose sum is σ = 37942926. Its totient is φ = 10840832.

The previous prime is 21681659. The next prime is 21681683. The reversal of 21681668 is 86618612.

It can be written as a sum of positive squares in only one way, i.e., 20775364 + 906304 = 4558^2 + 952^2 .

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 2710205 + ... + 2710212.

It is an arithmetic number, because the mean of its divisors is an integer number (6323821).

Almost surely, 2^21681668 is an apocalyptic number.

It is an amenable number.

21681668 is a deficient number, since it is larger than the sum of its proper divisors (16261258).

21681668 is a wasteful number, since it uses less digits than its factorization.

21681668 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5420421 (or 5420419 counting only the distinct ones).

The product of its digits is 27648, while the sum is 38.

The square root of 21681668 is about 4656.3578041212. The cubic root of 21681668 is about 278.8458786590.

The spelling of 21681668 in words is "twenty-one million, six hundred eighty-one thousand, six hundred sixty-eight".