21406531 has 2 divisors, whose sum is σ = 21406532. Its totient is φ = 21406530.

The previous prime is 21406529. The next prime is 21406543. The reversal of 21406531 is 13560412.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21406531 - 2¹ = 21406529 is a prime.

It is a super-2 number, since 2×21406531² = 916479138907922, which contains 22 as substring.

Together with 21406529, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21401531) 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 as a sum of consecutive naturals, namely, 10703265 + 10703266.

It is an arithmetic number, because the mean of its divisors is an integer number (10703266).

Almost surely, 2^21406531 is an apocalyptic number.

21406531 is a deficient number, since it is larger than the sum of its proper divisors (1).

21406531 is an equidigital number, since it uses as much as digits as its factorization.

21406531 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 720, while the sum is 22.

The square root of 21406531 is about 4626.7192480201. The cubic root of 21406531 is about 277.6613503127.

Adding to 21406531 its reverse (13560412), we get a palindrome (34966943).

The spelling of 21406531 in words is "twenty-one million, four hundred six thousand, five hundred thirty-one".