21211806 has 64 divisors (see below), whose sum is σ = 54028800. Its totient is φ = 5503680.

The previous prime is 21211783. The next prime is 21211807. The reversal of 21211806 is 60811212.

21211806 = 413² + 414² + ... + 511².

It is a Harshad number since it is a multiple of its sum of digits (21).

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21211807) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 22170 + ... + 23106.

It is an arithmetic number, because the mean of its divisors is an integer number (844200).

Almost surely, 2^21211806 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21211806, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27014400).

21211806 is an abundant number, since it is smaller than the sum of its proper divisors (32816994).

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

21211806 is a wasteful number, since it uses less digits than its factorization.

21211806 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 974 (or 960 counting only the distinct ones).

The product of its (nonzero) digits is 192, while the sum is 21.

The square root of 21211806 is about 4605.6276445236. The cubic root of 21211806 is about 276.8168668790.

The spelling of 21211806 in words is "twenty-one million, two hundred eleven thousand, eight hundred six".