99991561 has 2 divisors, whose sum is σ = 99991562. Its totient is φ = 99991560.

The previous prime is 99991559. The next prime is 99991583. The reversal of 99991561 is 16519999.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 90725625 + 9265936 = 9525^2 + 3044^2 .

It is an emirp because it is prime and its reverse (16519999) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 99991561 - 2¹ = 99991559 is a prime.

Together with 99991559, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (99941561) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 49995780 + 49995781.

It is an arithmetic number, because the mean of its divisors is an integer number (49995781).

Almost surely, 2^99991561 is an apocalyptic number.

It is an amenable number.

99991561 is a deficient number, since it is larger than the sum of its proper divisors (1).

99991561 is an equidigital number, since it uses as much as digits as its factorization.

99991561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 196830, while the sum is 49.

The square root of 99991561 is about 9999.5780410975. The cubic root of 99991561 is about 464.1458262046.

The spelling of 99991561 in words is "ninety-nine million, nine hundred ninety-one thousand, five hundred sixty-one".