21558851 has 4 divisors (see below), whose sum is σ = 21608400. Its totient is φ = 21509304.

The previous prime is 21558811. The next prime is 21558881. The reversal of 21558851 is 15885512.

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 21558851 - 2¹⁸ = 21296707 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21558811) 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, 24116 + ... + 24993.

It is an arithmetic number, because the mean of its divisors is an integer number (5402100).

Almost surely, 2^21558851 is an apocalyptic number.

21558851 is a deficient number, since it is larger than the sum of its proper divisors (49549).

21558851 is an equidigital number, since it uses as much as digits as its factorization.

21558851 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 49548.

The product of its digits is 16000, while the sum is 35.

The square root of 21558851 is about 4643.1509775152. The cubic root of 21558851 is about 278.3183687637.

It can be divided in two parts, 215 and 58851, that multiplied together give a triangular number (12652965 = T₅₀₃₀).

The spelling of 21558851 in words is "twenty-one million, five hundred fifty-eight thousand, eight hundred fifty-one".