21506553 has 10 divisors (see below), whose sum is σ = 32127194. Its totient is φ = 14337648.

The previous prime is 21506533. The next prime is 21506593. The reversal of 21506553 is 35560512.

21506553 is a `hidden beast` number, since 2 + 1 + 5 + 0 + 655 + 3 = 666.

It can be written as a sum of positive squares in only one way, i.e., 20820969 + 685584 = 4563^2 + 828^2 .

It is not a de Polignac number, because 21506553 - 2⁶ = 21506489 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21506533) by changing a digit.

It is a polite number, since it can be written in 9 ways as a sum of consecutive naturals, for example, 132676 + ... + 132837.

Almost surely, 2^21506553 is an apocalyptic number.

It is an amenable number.

21506553 is a deficient number, since it is larger than the sum of its proper divisors (10620641).

21506553 is an equidigital number, since it uses as much as digits as its factorization.

21506553 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 265525 (or 265516 counting only the distinct ones).

The product of its (nonzero) digits is 4500, while the sum is 27.

The square root of 21506553 is about 4637.5158220754. The cubic root of 21506553 is about 278.0931359868.

The spelling of 21506553 in words is "twenty-one million, five hundred six thousand, five hundred fifty-three".