21987540 has 144 divisors (see below), whose sum is σ = 71705088. Its totient is φ = 5440512.

The previous prime is 21987533. The next prime is 21987547. The reversal of 21987540 is 4578912.

21987540 is a `hidden beast` number, since 21 + 98 + 7 + 540 = 666.

It is an interprime number because it is at equal distance from previous prime (21987533) and next prime (21987547).

It is a Smith number, since the sum of its digits (36) coincides with the sum of the digits of its prime factors.

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

It is a nialpdrome in base 12.

It is a junction number, because it is equal to n+sod(n) for n = 21987495 and 21987504.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21987547) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 194524 + ... + 194636.

It is an arithmetic number, because the mean of its divisors is an integer number (497952).

Almost surely, 2^21987540 is an apocalyptic number.

21987540 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 21987540, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (35852544).

21987540 is an abundant number, since it is smaller than the sum of its proper divisors (49717548).

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

21987540 is a wasteful number, since it uses less digits than its factorization.

21987540 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 198 (or 193 counting only the distinct ones).

The product of its (nonzero) digits is 20160, while the sum is 36.

The square root of 21987540 is about 4689.0873312405. The cubic root of 21987540 is about 280.1510239695.

The spelling of 21987540 in words is "twenty-one million, nine hundred eighty-seven thousand, five hundred forty".