3991434021821 has 2 divisors, whose sum is σ = 3991434021822. Its totient is φ = 3991434021820.

The previous prime is 3991434021791. The next prime is 3991434021851. The reversal of 3991434021821 is 1281204341993.

It is a balanced prime because it is at equal distance from previous prime (3991434021791) and next prime (3991434021851).

It can be written as a sum of positive squares in only one way, i.e., 2625015636100 + 1366418385721 = 1620190^2 + 1168939^2 .

It is an emirp because it is prime and its reverse (1281204341993) is a distict prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-3991434021821 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 (3991434021851) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 1995717010910 + 1995717010911.

It is an arithmetic number, because the mean of its divisors is an integer number (1995717010911).

Almost surely, 2^{3991434021821} is an apocalyptic number.

It is an amenable number.

3991434021821 is a deficient number, since it is larger than the sum of its proper divisors (1).

3991434021821 is an equidigital number, since it uses as much as digits as its factorization.

3991434021821 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 373248, while the sum is 47.

The spelling of 3991434021821 in words is "three trillion, nine hundred ninety-one billion, four hundred thirty-four million, twenty-one thousand, eight hundred twenty-one".