21009906 has 12 divisors (see below), whose sum is σ = 45521502. Its totient is φ = 7003296.

The previous prime is 21009887. The next prime is 21009907. The reversal of 21009906 is 60990012.

It is a happy number.

It can be written as a sum of positive squares in only one way, i.e., 17715681 + 3294225 = 4209^2 + 1815^2 .

It is a self number, because there is not a number n which added to its sum of digits gives 21009906.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21009907) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 583591 + ... + 583626.

Almost surely, 2^21009906 is an apocalyptic number.

21009906 is an abundant number, since it is smaller than the sum of its proper divisors (24511596).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21009906 is a wasteful number, since it uses less digits than its factorization.

21009906 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1167225 (or 1167222 counting only the distinct ones).

The product of its (nonzero) digits is 972, while the sum is 27.

The square root of 21009906 is about 4583.6564007351. The cubic root of 21009906 is about 275.9357916170.

Adding to 21009906 its reverse (60990012), we get a palindrome (81999918).

The spelling of 21009906 in words is "twenty-one million, nine thousand, nine hundred six".