21515531 has 4 divisors (see below), whose sum is σ = 21528000. Its totient is φ = 21503064.

The previous prime is 21515521. The next prime is 21515551. The reversal of 21515531 is 13551512.

It is a happy number.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21515531 is a prime.

It is a super-2 number, since 2×21515531² = 925836148423922, 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 = 21515497 and 21515506.

It is not an unprimeable number, because it can be changed into a prime (21515521) 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, 3131 + ... + 7268.

It is an arithmetic number, because the mean of its divisors is an integer number (5382000).

Almost surely, 2^21515531 is an apocalyptic number.

21515531 is a deficient number, since it is larger than the sum of its proper divisors (12469).

21515531 is a wasteful number, since it uses less digits than its factorization.

21515531 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 12468.

The product of its digits is 750, while the sum is 23.

The square root of 21515531 is about 4638.4836962094. The cubic root of 21515531 is about 278.1318276487.

The spelling of 21515531 in words is "twenty-one million, five hundred fifteen thousand, five hundred thirty-one".