20805721 has 2 divisors, whose sum is σ = 20805722. Its totient is φ = 20805720.

The previous prime is 20805713. The next prime is 20805751. The reversal of 20805721 is 12750802.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15515721 + 5290000 = 3939^2 + 2300^2 .

It is a cyclic number.

It is not a de Polignac number, because 20805721 - 2³ = 20805713 is a prime.

It is equal to p_1318462 and since 20805721 and 1318462 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (20805751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10402860 + 10402861.

It is an arithmetic number, because the mean of its divisors is an integer number (10402861).

Almost surely, 2^20805721 is an apocalyptic number.

It is an amenable number.

20805721 is a deficient number, since it is larger than the sum of its proper divisors (1).

20805721 is an equidigital number, since it uses as much as digits as its factorization.

20805721 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1120, while the sum is 25.

The square root of 20805721 is about 4561.3288633906. The cubic root of 20805721 is about 275.0389851067.

The spelling of 20805721 in words is "twenty million, eight hundred five thousand, seven hundred twenty-one".