21251666 has 16 divisors (see below), whose sum is σ = 33957144. Its totient is φ = 9940224.

The previous prime is 21251617. The next prime is 21251669. The reversal of 21251666 is 66615212.

It can be written as a sum of positive squares in 4 ways, for example, as 18241441 + 3010225 = 4271^2 + 1735^2 .

It is not an unprimeable number, because it can be changed into a prime (21251669) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 4076 + ... + 7688.

Almost surely, 2^21251666 is an apocalyptic number.

21251666 is a deficient number, since it is larger than the sum of its proper divisors (12705478).

21251666 is a wasteful number, since it uses less digits than its factorization.

21251666 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3805.

The product of its digits is 4320, while the sum is 29.

The square root of 21251666 is about 4609.9529281762. The cubic root of 21251666 is about 276.9901511354.

Adding to 21251666 its reverse (66615212), we get a palindrome (87866878).

The spelling of 21251666 in words is "twenty-one million, two hundred fifty-one thousand, six hundred sixty-six".