21405420 has 72 divisors (see below), whose sum is σ = 65585520. Its totient is φ = 5650560.

The previous prime is 21405409. The next prime is 21405427. The reversal of 21405420 is 2450412.

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 = 21405393 and 21405402.

It is not an unprimeable number, because it can be changed into a prime (21405427) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 19075 + ... + 20165.

It is an arithmetic number, because the mean of its divisors is an integer number (910910).

Almost surely, 2^21405420 is an apocalyptic number.

21405420 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 21405420, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32792760).

21405420 is an abundant number, since it is smaller than the sum of its proper divisors (44180100).

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

21405420 is a wasteful number, since it uses less digits than its factorization.

21405420 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1215 (or 1210 counting only the distinct ones).

The product of its (nonzero) digits is 320, while the sum is 18.

The square root of 21405420 is about 4626.5991829853. The cubic root of 21405420 is about 277.6565466838.

Adding to 21405420 its reverse (2450412), we get a palindrome (23855832).

The spelling of 21405420 in words is "twenty-one million, four hundred five thousand, four hundred twenty".