21045906 has 80 divisors (see below), whose sum is σ = 54897216. Its totient is φ = 5901984.

The previous prime is 21045889. The next prime is 21045907. The reversal of 21045906 is 60954012.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a self number, because there is not a number n which added to its sum of digits gives 21045906.

It is not an unprimeable number, because it can be changed into a prime (21045907) by changing a digit.

It is a polite number, since it can be written in 39 ways as a sum of consecutive naturals, for example, 75840 + ... + 76116.

Almost surely, 2^21045906 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21045906, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27448608).

21045906 is an abundant number, since it is smaller than the sum of its proper divisors (33851310).

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

21045906 is a wasteful number, since it uses less digits than its factorization.

21045906 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 365 (or 356 counting only the distinct ones).

The product of its (nonzero) digits is 2160, while the sum is 27.

The square root of 21045906 is about 4587.5817158935. The cubic root of 21045906 is about 276.0933049376.

Adding to 21045906 its reverse (60954012), we get a palindrome (81999918).

The spelling of 21045906 in words is "twenty-one million, forty-five thousand, nine hundred six".