21205531 has 4 divisors (see below), whose sum is σ = 21348000. Its totient is φ = 21063064.

The previous prime is 21205529. The next prime is 21205543. The reversal of 21205531 is 13550212.

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 21205531 - 2¹ = 21205529 is a prime.

It is a super-2 number, since 2×21205531² = 899349089983922, which contains 22 as substring.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21205499 and 21205508.

It is not an unprimeable number, because it can be changed into a prime (21205231) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 71011 + ... + 71308.

It is an arithmetic number, because the mean of its divisors is an integer number (5337000).

Almost surely, 2^21205531 is an apocalyptic number.

21205531 is a deficient number, since it is larger than the sum of its proper divisors (142469).

21205531 is a wasteful number, since it uses less digits than its factorization.

21205531 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 142468.

The product of its (nonzero) digits is 300, while the sum is 19.

The square root of 21205531 is about 4604.9463623369. The cubic root of 21205531 is about 276.7895676609.

Adding to 21205531 its reverse (13550212), we get a palindrome (34755743).

It can be divided in two parts, 21205 and 531, that added together give a triangular number (21736 = T₂₀₈).

The spelling of 21205531 in words is "twenty-one million, two hundred five thousand, five hundred thirty-one".