21009551 has 4 divisors (see below), whose sum is σ = 21055392. Its totient is φ = 20963712.

The previous prime is 21009533. The next prime is 21009553. The reversal of 21009551 is 15590012.

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

It is a cyclic number.

It is not a de Polignac number, because 21009551 - 2¹⁰ = 21008527 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21009553) 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, 22226 + ... + 23151.

It is an arithmetic number, because the mean of its divisors is an integer number (5263848).

Almost surely, 2^21009551 is an apocalyptic number.

21009551 is a deficient number, since it is larger than the sum of its proper divisors (45841).

21009551 is an equidigital number, since it uses as much as digits as its factorization.

21009551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 45840.

The product of its (nonzero) digits is 450, while the sum is 23.

The square root of 21009551 is about 4583.6176760284. The cubic root of 21009551 is about 275.9342374651.

Adding to 21009551 its reverse (15590012), we get a palindrome (36599563).

It can be divided in two parts, 2 and 1009551, that multiplied together give a palindrome (2019102).

The spelling of 21009551 in words is "twenty-one million, nine thousand, five hundred fifty-one".