20811721 has 6 divisors (see below), whose sum is σ = 24209610. Its totient is φ = 17838576.

The previous prime is 20811691. The next prime is 20811731. The reversal of 20811721 is 12711802.

It can be written as a sum of positive squares in only one way, i.e., 20070400 + 741321 = 4480^2 + 861^2 .

It is not a de Polignac number, because 20811721 - 2²³ = 12423113 is a prime.

It is a Duffinian number.

It is a zygodrome in base 3.

It is a junction number, because it is equal to n+sod(n) for n = 20811692 and 20811701.

It is not an unprimeable number, because it can be changed into a prime (20811731) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 212316 + ... + 212413.

It is an arithmetic number, because the mean of its divisors is an integer number (4034935).

Almost surely, 2^20811721 is an apocalyptic number.

It is an amenable number.

20811721 is a deficient number, since it is larger than the sum of its proper divisors (3397889).

20811721 is an equidigital number, since it uses as much as digits as its factorization.

20811721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 424743 (or 424736 counting only the distinct ones).

The product of its (nonzero) digits is 224, while the sum is 22.

The square root of 20811721 is about 4561.9865190507. The cubic root of 20811721 is about 275.0654213499.

The spelling of 20811721 in words is "twenty million, eight hundred eleven thousand, seven hundred twenty-one".