21720906 has 16 divisors (see below), whose sum is σ = 48268800. Its totient is φ = 7240284.

The previous prime is 21720899. The next prime is 21720911. The reversal of 21720906 is 60902712.

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

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 21720906.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 201066 + ... + 201173.

It is an arithmetic number, because the mean of its divisors is an integer number (3016800).

Almost surely, 2^21720906 is an apocalyptic number.

21720906 is an abundant number, since it is smaller than the sum of its proper divisors (26547894).

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

21720906 is a wasteful number, since it uses less digits than its factorization.

21720906 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 402250 (or 402244 counting only the distinct ones).

The product of its (nonzero) digits is 1512, while the sum is 27.

The square root of 21720906 is about 4660.5692785324. The cubic root of 21720906 is about 279.0139893552.

The spelling of 21720906 in words is "twenty-one million, seven hundred twenty thousand, nine hundred six".