99991721 has 2 divisors, whose sum is σ = 99991722. Its totient is φ = 99991720.

The previous prime is 99991687. The next prime is 99991733. The reversal of 99991721 is 12719999.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 99780121 + 211600 = 9989^2 + 460^2 .

It is a cyclic number.

It is not a de Polignac number, because 99991721 - 2¹⁸ = 99729577 is a prime.

It is not a weakly prime, because it can be changed into another prime (99999721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 49995860 + 49995861.

It is an arithmetic number, because the mean of its divisors is an integer number (49995861).

Almost surely, 2^99991721 is an apocalyptic number.

It is an amenable number.

99991721 is a deficient number, since it is larger than the sum of its proper divisors (1).

99991721 is an equidigital number, since it uses as much as digits as its factorization.

99991721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 91854, while the sum is 47.

The square root of 99991721 is about 9999.5860414319. The cubic root of 99991721 is about 464.1460737698.

The spelling of 99991721 in words is "ninety-nine million, nine hundred ninety-one thousand, seven hundred twenty-one".