21213531 has 6 divisors (see below), whose sum is σ = 30641780. Its totient is φ = 14142348.

The previous prime is 21213487. The next prime is 21213553. The reversal of 21213531 is 13531212.

It is not a de Polignac number, because 21213531 - 2¹¹ = 21211483 is a prime.

It is a super-2 number, since 2×21213531² = 900027794975922, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21213431) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1178521 + ... + 1178538.

Almost surely, 2^21213531 is an apocalyptic number.

21213531 is a deficient number, since it is larger than the sum of its proper divisors (9428249).

21213531 is a wasteful number, since it uses less digits than its factorization.

21213531 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2357065 (or 2357062 counting only the distinct ones).

The product of its digits is 180, while the sum is 18.

The square root of 21213531 is about 4605.8149116090. The cubic root of 21213531 is about 276.8243705015.

Adding to 21213531 its reverse (13531212), we get a palindrome (34744743).

The spelling of 21213531 in words is "twenty-one million, two hundred thirteen thousand, five hundred thirty-one".