20721251 has 4 divisors (see below), whose sum is σ = 21112272. Its totient is φ = 20330232.

The previous prime is 20721223. The next prime is 20721263. The reversal of 20721251 is 15212702.

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 20721251 - 2¹⁸ = 20459107 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20721271) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 195431 + ... + 195536.

It is an arithmetic number, because the mean of its divisors is an integer number (5278068).

Almost surely, 2^20721251 is an apocalyptic number.

20721251 is a deficient number, since it is larger than the sum of its proper divisors (391021).

20721251 is an equidigital number, since it uses as much as digits as its factorization.

20721251 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 391020.

The product of its (nonzero) digits is 280, while the sum is 20.

The square root of 20721251 is about 4552.0600830833. The cubic root of 20721251 is about 274.6662662265. Note that the first 3 decimals are identical.

Adding to 20721251 its reverse (15212702), we get a palindrome (35933953).

The spelling of 20721251 in words is "twenty million, seven hundred twenty-one thousand, two hundred fifty-one".