21305531 has 16 divisors (see below), whose sum is σ = 23727648. Its totient is φ = 19008000.

The previous prime is 21305513. The next prime is 21305533. The reversal of 21305531 is 13550312.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21305531 is a prime.

It is a super-2 number, since 2×21305531² = 907851302383922, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21305533) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 52931 + ... + 53331.

It is an arithmetic number, because the mean of its divisors is an integer number (1482978).

Almost surely, 2^21305531 is an apocalyptic number.

21305531 is a deficient number, since it is larger than the sum of its proper divisors (2422117).

21305531 is a wasteful number, since it uses less digits than its factorization.

21305531 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 542.

The product of its (nonzero) digits is 450, while the sum is 20.

The square root of 21305531 is about 4615.7914814255. The cubic root of 21305531 is about 277.2239757308.

Adding to 21305531 its reverse (13550312), we get a palindrome (34855843).

The spelling of 21305531 in words is "twenty-one million, three hundred five thousand, five hundred thirty-one".