9921991 has 2 divisors, whose sum is σ = 9921992. Its totient is φ = 9921990.

The previous prime is 9921983. The next prime is 9921997. The reversal of 9921991 is 1991299.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 9921991 - 2³ = 9921983 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (9921997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 4960995 + 4960996.

It is an arithmetic number, because the mean of its divisors is an integer number (4960996).

Almost surely, 2^9921991 is an apocalyptic number.

9921991 is a deficient number, since it is larger than the sum of its proper divisors (1).

9921991 is an equidigital number, since it uses as much as digits as its factorization.

9921991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 13122, while the sum is 40.

The square root of 9921991 is about 3149.9192053131. The cubic root of 9921991 is about 214.8817882705.

The spelling of 9921991 in words is "nine million, nine hundred twenty-one thousand, nine hundred ninety-one".