1998405821 has 2 divisors, whose sum is σ = 1998405822. Its totient is φ = 1998405820.

The previous prime is 1998405797. The next prime is 1998405823. The reversal of 1998405821 is 1285048991.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1966922500 + 31483321 = 44350^2 + 5611^2 .

It is a cyclic number.

It is not a de Polignac number, because 1998405821 - 2²⁶ = 1931296957 is a prime.

Together with 1998405823, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1998405823) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 999202910 + 999202911.

It is an arithmetic number, because the mean of its divisors is an integer number (999202911).

Almost surely, 2^1998405821 is an apocalyptic number.

It is an amenable number.

1998405821 is a deficient number, since it is larger than the sum of its proper divisors (1).

1998405821 is an equidigital number, since it uses as much as digits as its factorization.

1998405821 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 207360, while the sum is 47.

The square root of 1998405821 is about 44703.5325337942. The cubic root of 1998405821 is about 1259.5862042986.

The spelling of 1998405821 in words is "one billion, nine hundred ninety-eight million, four hundred five thousand, eight hundred twenty-one".