643867991 has 8 divisors (see below), whose sum is σ = 729891120. Its totient is φ = 563058432.

The previous prime is 643867981. The next prime is 643867997. The reversal of 643867991 is 199768346.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 643867991 - 2¹⁰ = 643866967 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (643867997) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1303130 + ... + 1303623.

It is an arithmetic number, because the mean of its divisors is an integer number (91236390).

Almost surely, 2^643867991 is an apocalyptic number.

643867991 is a deficient number, since it is larger than the sum of its proper divisors (86023129).

643867991 is a wasteful number, since it uses less digits than its factorization.

643867991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2606785.

The product of its digits is 1959552, while the sum is 53.

The square root of 643867991 is about 25374.5540059328. The cubic root of 643867991 is about 863.5065014246.

The spelling of 643867991 in words is "six hundred forty-three million, eight hundred sixty-seven thousand, nine hundred ninety-one".