21561540 has 192 divisors, whose sum is σ = 81285120. Its totient is φ = 4124160.

The previous prime is 21561503. The next prime is 21561571. The reversal of 21561540 is 4516512.

It is a self number, because there is not a number n which added to its sum of digits gives 21561540.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 63 ways as a sum of consecutive naturals, for example, 59881 + ... + 60239.

It is an arithmetic number, because the mean of its divisors is an integer number (423360).

Almost surely, 2^21561540 is an apocalyptic number.

21561540 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 21561540, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (40642560).

21561540 is an abundant number, since it is smaller than the sum of its proper divisors (59723580).

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

21561540 is a wasteful number, since it uses less digits than its factorization.

21561540 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 402 (or 400 counting only the distinct ones).

The product of its (nonzero) digits is 1200, while the sum is 24.

The square root of 21561540 is about 4643.4405347759. The cubic root of 21561540 is about 278.3299396800.

The spelling of 21561540 in words is "twenty-one million, five hundred sixty-one thousand, five hundred forty".