21005531 has 4 divisors (see below), whose sum is σ = 21293352. Its totient is φ = 20717712.

The previous prime is 21005503. The next prime is 21005539. The reversal of 21005531 is 13550012.

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 21005531 - 2¹⁴ = 20989147 is a prime.

It is a super-2 number, since 2×21005531² = 882464665183922, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21005539) 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, 143801 + ... + 143946.

It is an arithmetic number, because the mean of its divisors is an integer number (5323338).

Almost surely, 2^21005531 is an apocalyptic number.

21005531 is a deficient number, since it is larger than the sum of its proper divisors (287821).

21005531 is an equidigital number, since it uses as much as digits as its factorization.

21005531 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 287820.

The product of its (nonzero) digits is 150, while the sum is 17.

The square root of 21005531 is about 4583.1791368001. The cubic root of 21005531 is about 275.9166371145.

Adding to 21005531 its reverse (13550012), we get a palindrome (34555543).

The spelling of 21005531 in words is "twenty-one million, five thousand, five hundred thirty-one".