20856721 has 2 divisors, whose sum is σ = 20856722. Its totient is φ = 20856720.

The previous prime is 20856709. The next prime is 20856733. The reversal of 20856721 is 12765802.

It is a balanced prime because it is at equal distance from previous prime (20856709) and next prime (20856733).

It can be written as a sum of positive squares in only one way, i.e., 20151121 + 705600 = 4489^2 + 840^2 .

It is a cyclic number.

It is not a de Polignac number, because 20856721 - 2²¹ = 18759569 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (20856751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10428360 + 10428361.

It is an arithmetic number, because the mean of its divisors is an integer number (10428361).

Almost surely, 2^20856721 is an apocalyptic number.

It is an amenable number.

20856721 is a deficient number, since it is larger than the sum of its proper divisors (1).

20856721 is an equidigital number, since it uses as much as digits as its factorization.

20856721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 6720, while the sum is 31.

The square root of 20856721 is about 4566.9159177721. The cubic root of 20856721 is about 275.2635314003.

The spelling of 20856721 in words is "twenty million, eight hundred fifty-six thousand, seven hundred twenty-one".