21506220 has 72 divisors (see below), whose sum is σ = 65725296. Its totient is φ = 5692032.

The previous prime is 21506209. The next prime is 21506257. The reversal of 21506220 is 2260512.

It is a Harshad number since it is a multiple of its sum of digits (18).

It is a junction number, because it is equal to n+sod(n) for n = 21506193 and 21506202.

It is an unprimeable number.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 28974 + ... + 29706.

Almost surely, 2^21506220 is an apocalyptic number.

21506220 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21506220, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32862648).

21506220 is an abundant number, since it is smaller than the sum of its proper divisors (44219076).

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

21506220 is a wasteful number, since it uses less digits than its factorization.

21506220 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 911 (or 906 counting only the distinct ones).

The product of its (nonzero) digits is 240, while the sum is 18.

The square root of 21506220 is about 4637.4799190940. The cubic root of 21506220 is about 278.0917006802.

Adding to 21506220 its reverse (2260512), we get a palindrome (23766732).

The spelling of 21506220 in words is "twenty-one million, five hundred six thousand, two hundred twenty".