21651840 has 256 divisors, whose sum is σ = 88128000. Its totient is φ = 4921344.

The previous prime is 21651823. The next prime is 21651841. The reversal of 21651840 is 4815612.

21651840 is a `hidden beast` number, since 2 + 1 + 651 + 8 + 4 + 0 = 666.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21651841) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 120871 + ... + 121049.

It is an arithmetic number, because the mean of its divisors is an integer number (344250).

Almost surely, 2^21651840 is an apocalyptic number.

21651840 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 21651840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (44064000).

21651840 is an abundant number, since it is smaller than the sum of its proper divisors (66476160).

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

21651840 is a wasteful number, since it uses less digits than its factorization.

21651840 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 214 (or 196 counting only the distinct ones).

The product of its (nonzero) digits is 1920, while the sum is 27.

The square root of 21651840 is about 4653.1537692193. The cubic root of 21651840 is about 278.7179482810.

It can be divided in two parts, 2165 and 1840, that added together give a triangular number (4005 = T₈₉).

The spelling of 21651840 in words is "twenty-one million, six hundred fifty-one thousand, eight hundred forty".