21581247 has 4 divisors (see below), whose sum is σ = 28775000. Its totient is φ = 14387496.

The previous prime is 21581233. The next prime is 21581257. The reversal of 21581247 is 74218512.

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

It is not a de Polignac number, because 21581247 - 2¹¹ = 21579199 is a prime.

It is a Duffinian number.

It is a plaindrome in base 8.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21581257) 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, 3596872 + ... + 3596877.

It is an arithmetic number, because the mean of its divisors is an integer number (7193750).

Almost surely, 2^21581247 is an apocalyptic number.

21581247 is a deficient number, since it is larger than the sum of its proper divisors (7193753).

21581247 is an equidigital number, since it uses as much as digits as its factorization.

21581247 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7193752.

The product of its digits is 4480, while the sum is 30.

The square root of 21581247 is about 4645.5620757880. The cubic root of 21581247 is about 278.4147106484.

Adding to 21581247 its reverse (74218512), we get a palindrome (95799759).

The spelling of 21581247 in words is "twenty-one million, five hundred eighty-one thousand, two hundred forty-seven".