21521540 has 12 divisors (see below), whose sum is σ = 45195276. Its totient is φ = 8608608.

The previous prime is 21521527. The next prime is 21521557. The reversal of 21521540 is 4512512.

It can be written as a sum of positive squares in 2 ways, for example, as 2876416 + 18645124 = 1696^2 + 4318^2 .

It is a Harshad number since it is a multiple of its sum of digits (20), and also a Moran number because the ratio is a prime number: 1076077 = 21521540 / (2 + 1 + 5 + 2 + 1 + 5 + 4 + 0).

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, 538019 + ... + 538058.

It is an arithmetic number, because the mean of its divisors is an integer number (3766273).

Almost surely, 2^21521540 is an apocalyptic number.

21521540 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21521540 is an abundant number, since it is smaller than the sum of its proper divisors (23673736).

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

21521540 is a wasteful number, since it uses less digits than its factorization.

21521540 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1076086 (or 1076084 counting only the distinct ones).

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

The square root of 21521540 is about 4639.1313842141. The cubic root of 21521540 is about 278.1577180716.

Subtracting from 21521540 its reverse (4512512), we obtain a triangular number (17009028 = T₅₈₃₂).

The spelling of 21521540 in words is "twenty-one million, five hundred twenty-one thousand, five hundred forty".