1991998621 has 2 divisors, whose sum is σ = 1991998622. Its totient is φ = 1991998620.

The previous prime is 1991998609. The next prime is 1991998633. The reversal of 1991998621 is 1268991991.

It is a balanced prime because it is at equal distance from previous prime (1991998609) and next prime (1991998633).

It can be written as a sum of positive squares in only one way, i.e., 1273490596 + 718508025 = 35686^2 + 26805^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-1991998621 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1991996621) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 995999310 + 995999311.

It is an arithmetic number, because the mean of its divisors is an integer number (995999311).

Almost surely, 2^1991998621 is an apocalyptic number.

It is an amenable number.

1991998621 is a deficient number, since it is larger than the sum of its proper divisors (1).

1991998621 is an equidigital number, since it uses as much as digits as its factorization.

1991998621 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 629856, while the sum is 55.

The square root of 1991998621 is about 44631.8117602232. The cubic root of 1991998621 is about 1258.2386199646.

The spelling of 1991998621 in words is "one billion, nine hundred ninety-one million, nine hundred ninety-eight thousand, six hundred twenty-one".