21000040 has 16 divisors (see below), whose sum is σ = 47250180. Its totient is φ = 8400000.

The previous prime is 21000037. The next prime is 21000041. The reversal of 21000040 is 4000012.

21000040 is an esthetic number in base 9, because in such base its adjacent digits differ by 1.

It can be written as a sum of positive squares in 2 ways, for example, as 20775364 + 224676 = 4558^2 + 474^2 .

It is a junction number, because it is equal to n+sod(n) for n = 20999987 and 21000032.

It is not an unprimeable number, because it can be changed into a prime (21000041) 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 3 ways as a sum of consecutive naturals, for example, 262461 + ... + 262540.

Almost surely, 2^21000040 is an apocalyptic number.

21000040 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21000040 is an abundant number, since it is smaller than the sum of its proper divisors (26250140).

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

21000040 is a wasteful number, since it uses less digits than its factorization.

21000040 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 525012 (or 525008 counting only the distinct ones).

The product of its (nonzero) digits is 8, while the sum is 7.

The square root of 21000040 is about 4582.5800593116. The cubic root of 21000040 is about 275.8925928078.

Adding to 21000040 its reverse (4000012), we get a palindrome (25000052).

The spelling of 21000040 in words is "twenty-one million, forty", and thus it is an aban number and an uban number.