21007666 has 4 divisors (see below), whose sum is σ = 31511502. Its totient is φ = 10503832.

The previous prime is 21007639. The next prime is 21007669. The reversal of 21007666 is 66670012.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in only one way, i.e., 13905441 + 7102225 = 3729^2 + 2665^2 .

It is not an unprimeable number, because it can be changed into a prime (21007669) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5251915 + ... + 5251918.

Almost surely, 2^21007666 is an apocalyptic number.

21007666 is a deficient number, since it is larger than the sum of its proper divisors (10503836).

21007666 is a wasteful number, since it uses less digits than its factorization.

21007666 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10503835.

The product of its (nonzero) digits is 3024, while the sum is 28.

The square root of 21007666 is about 4583.4120478089. The cubic root of 21007666 is about 275.9259848440.

Adding to 21007666 its reverse (66670012), we get a palindrome (87677678).

The spelling of 21007666 in words is "twenty-one million, seven thousand, six hundred sixty-six".