1991398021 has 2 divisors, whose sum is σ = 1991398022. Its totient is φ = 1991398020.

The previous prime is 1991398007. The next prime is 1991398027. The reversal of 1991398021 is 1208931991.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1991390625 + 7396 = 44625^2 + 86^2 .

It is a cyclic number.

It is not a de Polignac number, because 1991398021 - 2¹⁹ = 1990873733 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1991398027) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 995699010 + 995699011.

It is an arithmetic number, because the mean of its divisors is an integer number (995699011).

Almost surely, 2^1991398021 is an apocalyptic number.

It is an amenable number.

1991398021 is a deficient number, since it is larger than the sum of its proper divisors (1).

1991398021 is an equidigital number, since it uses as much as digits as its factorization.

1991398021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 34992, while the sum is 43.

The square root of 1991398021 is about 44625.0828682704. The cubic root of 1991398021 is about 1258.1121516580.

The spelling of 1991398021 in words is "one billion, nine hundred ninety-one million, three hundred ninety-eight thousand, twenty-one".