21361721809 has 2 divisors, whose sum is σ = 21361721810. Its totient is φ = 21361721808.

The previous prime is 21361721789. The next prime is 21361721819. The reversal of 21361721809 is 90812716312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20821624209 + 540097600 = 144297^2 + 23240^2 .

It is a cyclic number.

It is not a de Polignac number, because 21361721809 - 2¹¹ = 21361719761 is a prime.

It is not a weakly prime, because it can be changed into another prime (21361721819) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10680860904 + 10680860905.

It is an arithmetic number, because the mean of its divisors is an integer number (10680860905).

Almost surely, 2^21361721809 is an apocalyptic number.

It is an amenable number.

21361721809 is a deficient number, since it is larger than the sum of its proper divisors (1).

21361721809 is an equidigital number, since it uses as much as digits as its factorization.

21361721809 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 36288, while the sum is 40.

The spelling of 21361721809 in words is "twenty-one billion, three hundred sixty-one million, seven hundred twenty-one thousand, eight hundred nine".