16991561 has 2 divisors, whose sum is σ = 16991562. Its totient is φ = 16991560.

The previous prime is 16991537. The next prime is 16991591. The reversal of 16991561 is 16519961.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 16974400 + 17161 = 4120^2 + 131^2 .

It is an emirp because it is prime and its reverse (16519961) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 16991561 - 2⁶ = 16991497 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (16991501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 8495780 + 8495781.

It is an arithmetic number, because the mean of its divisors is an integer number (8495781).

Almost surely, 2^16991561 is an apocalyptic number.

It is an amenable number.

16991561 is a deficient number, since it is larger than the sum of its proper divisors (1).

16991561 is an equidigital number, since it uses as much as digits as its factorization.

16991561 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 14580, while the sum is 38.

The square root of 16991561 is about 4122.0821195119. The cubic root of 16991561 is about 257.0856048758.

The spelling of 16991561 in words is "sixteen million, nine hundred ninety-one thousand, five hundred sixty-one".