21005540 has 24 divisors (see below), whose sum is σ = 46707192. Its totient is φ = 7907840.

The previous prime is 21005539. The next prime is 21005561. The reversal of 21005540 is 4550012.

It can be written as a sum of positive squares in 4 ways, for example, as 357604 + 20647936 = 598^2 + 4544^2 .

It is a Harshad number since it is a multiple of its sum of digits (17).

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 30551 + ... + 31230.

It is an arithmetic number, because the mean of its divisors is an integer number (1946133).

Almost surely, 2^21005540 is an apocalyptic number.

21005540 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21005540 is an abundant number, since it is smaller than the sum of its proper divisors (25701652).

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

21005540 is a wasteful number, since it uses less digits than its factorization.

21005540 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 61807 (or 61805 counting only the distinct ones).

The product of its (nonzero) digits is 200, while the sum is 17.

The square root of 21005540 is about 4583.1801186512. The cubic root of 21005540 is about 275.9166765208.

Adding to 21005540 its reverse (4550012), we get a palindrome (25555552).

The spelling of 21005540 in words is "twenty-one million, five thousand, five hundred forty".