21137521 has 4 divisors (see below), whose sum is σ = 21167092. Its totient is φ = 21107952.

The previous prime is 21137507. The next prime is 21137531. The reversal of 21137521 is 12573112.

It is a happy number.

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

It can be written as a sum of positive squares in 2 ways, for example, as 17969121 + 3168400 = 4239^2 + 1780^2 .

It is a cyclic number.

It is not a de Polignac number, because 21137521 - 2⁵ = 21137489 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21137492 and 21137501.

It is not an unprimeable number, because it can be changed into a prime (21137531) 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, 13686 + ... + 15151.

It is an arithmetic number, because the mean of its divisors is an integer number (5291773).

Almost surely, 2^21137521 is an apocalyptic number.

It is an amenable number.

21137521 is a deficient number, since it is larger than the sum of its proper divisors (29571).

21137521 is an equidigital number, since it uses as much as digits as its factorization.

21137521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 29570.

The product of its digits is 420, while the sum is 22.

The square root of 21137521 is about 4597.5559811709. Note that the first 3 decimals coincide. The cubic root of 21137521 is about 276.4933459045.

The spelling of 21137521 in words is "twenty-one million, one hundred thirty-seven thousand, five hundred twenty-one".