20687921 has 4 divisors (see below), whose sum is σ = 21247092. Its totient is φ = 20128752.

The previous prime is 20687897. The next prime is 20687939. The reversal of 20687921 is 12978602.

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 13424896 + 7263025 = 3664^2 + 2695^2 .

It is a cyclic number.

It is not a de Polignac number, because 20687921 - 2⁶ = 20687857 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20687951) 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, 279530 + ... + 279603.

It is an arithmetic number, because the mean of its divisors is an integer number (5311773).

Almost surely, 2^20687921 is an apocalyptic number.

It is an amenable number.

20687921 is a deficient number, since it is larger than the sum of its proper divisors (559171).

20687921 is an equidigital number, since it uses as much as digits as its factorization.

20687921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 559170.

The product of its (nonzero) digits is 12096, while the sum is 35.

The square root of 20687921 is about 4548.3976299352. The cubic root of 20687921 is about 274.5189208847.

The spelling of 20687921 in words is "twenty million, six hundred eighty-seven thousand, nine hundred twenty-one".