20217721 has 4 divisors (see below), whose sum is σ = 20461392. Its totient is φ = 19974052.

The previous prime is 20217679. The next prime is 20217733. The reversal of 20217721 is 12771202.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 12771202 = 2 ⋅6385601.

It is a cyclic number.

It is not a de Polignac number, because 20217721 - 2⁹ = 20217209 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 20217692 and 20217701.

It is not an unprimeable number, because it can be changed into a prime (20217761) 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, 121711 + ... + 121876.

It is an arithmetic number, because the mean of its divisors is an integer number (5115348).

Almost surely, 2^20217721 is an apocalyptic number.

It is an amenable number.

20217721 is a deficient number, since it is larger than the sum of its proper divisors (243671).

20217721 is an equidigital number, since it uses as much as digits as its factorization.

20217721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 243670.

The product of its (nonzero) digits is 392, while the sum is 22.

The square root of 20217721 is about 4496.4120140396. The cubic root of 20217721 is about 272.4231851495.

Adding to 20217721 its reverse (12771202), we get a palindrome (32988923).

The spelling of 20217721 in words is "twenty million, two hundred seventeen thousand, seven hundred twenty-one".