21551240 has 64 divisors (see below), whose sum is σ = 52662960. Its totient is φ = 7905280.

The previous prime is 21551221. The next prime is 21551251. The reversal of 21551240 is 4215512.

It can be written as a sum of positive squares in 8 ways, for example, as 2067844 + 19483396 = 1438^2 + 4414^2 .

It is a Harshad number since it is a multiple of its sum of digits (20).

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 27494 + ... + 28266.

Almost surely, 2^21551240 is an apocalyptic number.

21551240 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 21551240, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26331480).

21551240 is an abundant number, since it is smaller than the sum of its proper divisors (31111720).

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

21551240 is a wasteful number, since it uses less digits than its factorization.

21551240 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 842 (or 838 counting only the distinct ones).

The product of its (nonzero) digits is 400, while the sum is 20.

The square root of 21551240 is about 4642.3313108825. The cubic root of 21551240 is about 278.2856129916.

Adding to 21551240 its reverse (4215512), we get a palindrome (25766752).

The spelling of 21551240 in words is "twenty-one million, five hundred fifty-one thousand, two hundred forty".