21561984 has 128 divisors (see below), whose sum is σ = 67564800. Its totient is φ = 6746112.

The previous prime is 21561949. The next prime is 21561997. The reversal of 21561984 is 48916512.

21561984 is a `hidden beast` number, since 2 + 1 + 561 + 98 + 4 = 666.

It is a tau number, because it is divible by the number of its divisors (128).

It is a Harshad number since it is a multiple of its sum of digits (36).

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, 58569 + ... + 58935.

It is an arithmetic number, because the mean of its divisors is an integer number (527850).

Almost surely, 2^21561984 is an apocalyptic number.

21561984 is a gapful number since it is divisible by the number (24) 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 21561984, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (33782400).

21561984 is an abundant number, since it is smaller than the sum of its proper divisors (46002816).

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

21561984 is a wasteful number, since it uses less digits than its factorization.

21561984 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 407 (or 389 counting only the distinct ones).

The product of its digits is 17280, while the sum is 36.

The square root of 21561984 is about 4643.4883439070. The cubic root of 21561984 is about 278.3318501441.

The spelling of 21561984 in words is "twenty-one million, five hundred sixty-one thousand, nine hundred eighty-four".