21017920 has 112 divisors (see below), whose sum is σ = 62471808. Its totient is φ = 6543360.

The previous prime is 21017897. The next prime is 21017951. The reversal of 21017920 is 2971012.

It is a tau number, because it is divible by the number of its divisors (112).

It is a super-2 number, since 2×21017920² = 883505922252800, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (22).

It is a junction number, because it is equal to n+sod(n) for n = 21017891 and 21017900.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 24214 + ... + 25066.

It is an arithmetic number, because the mean of its divisors is an integer number (557784).

Almost surely, 2^21017920 is an apocalyptic number.

21017920 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 21017920, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (31235904).

21017920 is an abundant number, since it is smaller than the sum of its proper divisors (41453888).

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

21017920 is a wasteful number, since it uses less digits than its factorization.

21017920 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 888 (or 878 counting only the distinct ones).

The product of its (nonzero) digits is 252, while the sum is 22.

The square root of 21017920 is about 4584.5305103140. The cubic root of 21017920 is about 275.9708713921.

Adding to 21017920 its reverse (2971012), we get a palindrome (23988932).

The spelling of 21017920 in words is "twenty-one million, seventeen thousand, nine hundred twenty".