21005521 has 4 divisors (see below), whose sum is σ = 21024052. Its totient is φ = 20986992.

The previous prime is 21005503. The next prime is 21005539. The reversal of 21005521 is 12550012.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21005503) and next prime (21005539).

It can be written as a sum of positive squares in 2 ways, for example, as 8473921 + 12531600 = 2911^2 + 3540^2 .

It is a cyclic number.

It is not a de Polignac number, because 21005521 - 2⁹ = 21005009 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21005495 and 21005504.

It is not an unprimeable number, because it can be changed into a prime (21005561) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 7446 + ... + 9871.

It is an arithmetic number, because the mean of its divisors is an integer number (5256013).

Almost surely, 2^21005521 is an apocalyptic number.

It is an amenable number.

21005521 is a deficient number, since it is larger than the sum of its proper divisors (18531).

21005521 is a wasteful number, since it uses less digits than its factorization.

21005521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 18530.

The product of its (nonzero) digits is 100, while the sum is 16.

The square root of 21005521 is about 4583.1780458542. The cubic root of 21005521 is about 275.9165933297.

Adding to 21005521 its reverse (12550012), we get a palindrome (33555533).

The spelling of 21005521 in words is "twenty-one million, five thousand, five hundred twenty-one".