21781540 has 48 divisors (see below), whose sum is σ = 50266944. Its totient is φ = 7862400.

The previous prime is 21781531. The next prime is 21781547. The reversal of 21781540 is 4518712.

It is a junction number, because it is equal to n+sod(n) for n = 21781499 and 21781508.

It is not an unprimeable number, because it can be changed into a prime (21781547) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 39547 + ... + 40093.

It is an arithmetic number, because the mean of its divisors is an integer number (1047228).

Almost surely, 2^21781540 is an apocalyptic number.

21781540 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 21781540, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25133472).

21781540 is an abundant number, since it is smaller than the sum of its proper divisors (28485404).

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

21781540 is a wasteful number, since it uses less digits than its factorization.

21781540 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 748 (or 746 counting only the distinct ones).

The product of its (nonzero) digits is 2240, while the sum is 28.

The square root of 21781540 is about 4667.0697444971. The cubic root of 21781540 is about 279.2733710560.

The spelling of 21781540 in words is "twenty-one million, seven hundred eighty-one thousand, five hundred forty".