991721641 has 2 divisors, whose sum is σ = 991721642. Its totient is φ = 991721640.

The previous prime is 991721629. The next prime is 991721669. The reversal of 991721641 is 146127199.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 917847616 + 73874025 = 30296^2 + 8595^2 .

It is a cyclic number.

It is not a de Polignac number, because 991721641 - 2¹¹ = 991719593 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 991721594 and 991721603.

It is not a weakly prime, because it can be changed into another prime (991721611) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495860820 + 495860821.

It is an arithmetic number, because the mean of its divisors is an integer number (495860821).

Almost surely, 2^991721641 is an apocalyptic number.

It is an amenable number.

991721641 is a deficient number, since it is larger than the sum of its proper divisors (1).

991721641 is an equidigital number, since it uses as much as digits as its factorization.

991721641 is an evil number, because the sum of its binary digits is even.

The product of its digits is 27216, while the sum is 40.

The square root of 991721641 is about 31491.6122324660. The cubic root of 991721641 is about 997.2328972046.

The spelling of 991721641 in words is "nine hundred ninety-one million, seven hundred twenty-one thousand, six hundred forty-one".