21187521 has 8 divisors (see below), whose sum is σ = 31388960. Its totient is φ = 14124996.

The previous prime is 21187511. The next prime is 21187561. The reversal of 21187521 is 12578112.

It is not a de Polignac number, because 21187521 - 2¹¹ = 21185473 is a prime.

It is a super-2 number, since 2×21187521² = 897822092250882, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 784723 = 21187521 / (2 + 1 + 1 + 8 + 7 + 5 + 2 + 1).

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21187511) 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 in 7 ways as a sum of consecutive naturals, for example, 392335 + ... + 392388.

It is an arithmetic number, because the mean of its divisors is an integer number (3923620).

Almost surely, 2^21187521 is an apocalyptic number.

It is an amenable number.

21187521 is a deficient number, since it is larger than the sum of its proper divisors (10201439).

21187521 is an equidigital number, since it uses as much as digits as its factorization.

21187521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 784732 (or 784726 counting only the distinct ones).

The product of its digits is 1120, while the sum is 27.

The square root of 21187521 is about 4602.9904410068. The cubic root of 21187521 is about 276.7111857199.

The spelling of 21187521 in words is "twenty-one million, one hundred eighty-seven thousand, five hundred twenty-one".