6561991 has 2 divisors, whose sum is σ = 6561992. Its totient is φ = 6561990.

The previous prime is 6561943. The next prime is 6562001. The reversal of 6561991 is 1991656.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 6561991 - 2¹⁵ = 6529223 is a prime.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 6561991.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (6561911) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 3280995 + 3280996.

It is an arithmetic number, because the mean of its divisors is an integer number (3280996).

Almost surely, 2^6561991 is an apocalyptic number.

6561991 is a deficient number, since it is larger than the sum of its proper divisors (1).

6561991 is an equidigital number, since it uses as much as digits as its factorization.

6561991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 14580, while the sum is 37.

The square root of 6561991 is about 2561.6383429360. The cubic root of 6561991 is about 187.2169691286.

The spelling of 6561991 in words is "six million, five hundred sixty-one thousand, nine hundred ninety-one".