20211209991821 has 2 divisors, whose sum is σ = 20211209991822. Its totient is φ = 20211209991820.

The previous prime is 20211209991793. The next prime is 20211209991839. The reversal of 20211209991821 is 12819990211202.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15550490314921 + 4660719676900 = 3943411^2 + 2158870^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20211209991821 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20211209991121) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10105604995910 + 10105604995911.

It is an arithmetic number, because the mean of its divisors is an integer number (10105604995911).

Almost surely, 2^{20211209991821} is an apocalyptic number.

It is an amenable number.

20211209991821 is a deficient number, since it is larger than the sum of its proper divisors (1).

20211209991821 is an equidigital number, since it uses as much as digits as its factorization.

20211209991821 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 93312, while the sum is 47.

The spelling of 20211209991821 in words is "twenty trillion, two hundred eleven billion, two hundred nine million, nine hundred ninety-one thousand, eight hundred twenty-one".