20215721 has 2 divisors, whose sum is σ = 20215722. Its totient is φ = 20215720.

The previous prime is 20215711. The next prime is 20215739. The reversal of 20215721 is 12751202.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20025625 + 190096 = 4475^2 + 436^2 .

It is a cyclic number.

It is not a de Polignac number, because 20215721 - 2²² = 16021417 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 20215693 and 20215702.

It is not a weakly prime, because it can be changed into another prime (20215711) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10107860 + 10107861.

It is an arithmetic number, because the mean of its divisors is an integer number (10107861).

Almost surely, 2^20215721 is an apocalyptic number.

It is an amenable number.

20215721 is a deficient number, since it is larger than the sum of its proper divisors (1).

20215721 is an equidigital number, since it uses as much as digits as its factorization.

20215721 is an evil number, because the sum of its binary digits is even.

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

The square root of 20215721 is about 4496.1896089912. The cubic root of 20215721 is about 272.4142018697.

Adding to 20215721 its reverse (12751202), we get a palindrome (32966923).

The spelling of 20215721 in words is "twenty million, two hundred fifteen thousand, seven hundred twenty-one".