21665280 has 320 divisors, whose sum is σ = 87994368. Its totient is φ = 4423680.

The previous prime is 21665279. The next prime is 21665341. The reversal of 21665280 is 8256612.

It is a tau number, because it is divible by the number of its divisors (320).

It is a Harshad number since it is a multiple of its sum of digits (30).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 698865 + ... + 698895.

Almost surely, 2^21665280 is an apocalyptic number.

21665280 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 21665280, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (43997184).

21665280 is an abundant number, since it is smaller than the sum of its proper divisors (66329088).

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

21665280 is a wasteful number, since it uses less digits than its factorization.

21665280 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 77 (or 61 counting only the distinct ones).

The product of its (nonzero) digits is 5760, while the sum is 30.

The square root of 21665280 is about 4654.5977269792. The cubic root of 21665280 is about 278.7756061123.

It can be divided in two parts, 21665 and 280, that added together give a triangular number (21945 = T₂₀₉).

The spelling of 21665280 in words is "twenty-one million, six hundred sixty-five thousand, two hundred eighty".