21506440 has 16 divisors (see below), whose sum is σ = 48389580. Its totient is φ = 8602560.

The previous prime is 21506437. The next prime is 21506453. The reversal of 21506440 is 4460512.

It can be written as a sum of positive squares in 2 ways, for example, as 20412324 + 1094116 = 4518^2 + 1046^2 .

It is a super-2 number, since 2×21506440² = 925053922947200, which contains 22 as substring.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 268791 + ... + 268870.

Almost surely, 2^21506440 is an apocalyptic number.

21506440 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21506440 is an abundant number, since it is smaller than the sum of its proper divisors (26883140).

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

21506440 is a wasteful number, since it uses less digits than its factorization.

21506440 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 537672 (or 537668 counting only the distinct ones).

The product of its (nonzero) digits is 960, while the sum is 22.

The square root of 21506440 is about 4637.5036388126. The cubic root of 21506440 is about 278.0926489325.

Adding to 21506440 its reverse (4460512), we get a palindrome (25966952).

The spelling of 21506440 in words is "twenty-one million, five hundred six thousand, four hundred forty".