21006720 has 192 divisors, whose sum is σ = 83060640. Its totient is φ = 4792320.

The previous prime is 21006707. The next prime is 21006731. The reversal of 21006720 is 2760012.

21006720 = T₄₈₉ + T₄₉₀ + ... + T₆₂₃.

It is a happy number.

It is a tau number, because it is divible by the number of its divisors (192).

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 = 21006693 and 21006702.

It is a congruent number.

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, 40060 + ... + 40580.

Almost surely, 2^21006720 is an apocalyptic number.

21006720 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 21006720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (41530320).

21006720 is an abundant number, since it is smaller than the sum of its proper divisors (62053920).

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

21006720 is a wasteful number, since it uses less digits than its factorization.

21006720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 553 (or 538 counting only the distinct ones).

The product of its (nonzero) digits is 168, while the sum is 18.

The square root of 21006720 is about 4583.3088484194. The cubic root of 21006720 is about 275.9218430242.

Adding to 21006720 its reverse (2760012), we get a palindrome (23766732).

The spelling of 21006720 in words is "twenty-one million, six thousand, seven hundred twenty".