21551861 has 4 divisors (see below), whose sum is σ = 21689292. Its totient is φ = 21414432.

The previous prime is 21551819. The next prime is 21551917. The reversal of 21551861 is 16815512.

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

It can be written as a sum of positive squares in 2 ways, for example, as 14364100 + 7187761 = 3790^2 + 2681^2 .

It is a cyclic number.

It is not a de Polignac number, because 21551861 - 2¹⁴ = 21535477 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 (21551561) 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, 68480 + ... + 68793.

It is an arithmetic number, because the mean of its divisors is an integer number (5422323).

Almost surely, 2^21551861 is an apocalyptic number.

It is an amenable number.

21551861 is a deficient number, since it is larger than the sum of its proper divisors (137431).

21551861 is a wasteful number, since it uses less digits than its factorization.

21551861 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 137430.

The product of its digits is 2400, while the sum is 29.

The square root of 21551861 is about 4642.3981948988. The cubic root of 21551861 is about 278.2882859037.

The spelling of 21551861 in words is "twenty-one million, five hundred fifty-one thousand, eight hundred sixty-one".