9991561 has 4 divisors (see below), whose sum is σ = 10009872. Its totient is φ = 9973252.

The previous prime is 9991559. The next prime is 9991567. The reversal of 9991561 is 1651999.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 1651999 = 281 ⋅5879.

It is a cyclic number.

It is not a de Polignac number, because 9991561 - 2¹ = 9991559 is a prime.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

9991561 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (9991567) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 8311 + ... + 9436.

It is an arithmetic number, because the mean of its divisors is an integer number (2502468).

Almost surely, 2^9991561 is an apocalyptic number.

It is an amenable number.

9991561 is a deficient number, since it is larger than the sum of its proper divisors (18311).

9991561 is a wasteful number, since it uses less digits than its factorization.

9991561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 18310.

The product of its digits is 21870, while the sum is 40.

The square root of 9991561 is about 3160.9430554820. The cubic root of 9991561 is about 215.3828476994.

The spelling of 9991561 in words is "nine million, nine hundred ninety-one thousand, five hundred sixty-one".