66801991 has 4 divisors (see below), whose sum is σ = 66869992. Its totient is φ = 66733992.

The previous prime is 66801979. The next prime is 66802007. The reversal of 66801991 is 19910866.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-66801991 is a prime.

It is a Duffinian number.

It is a zygodrome in base 3.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (66801901) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 32505 + ... + 34498.

It is an arithmetic number, because the mean of its divisors is an integer number (16717498).

Almost surely, 2^66801991 is an apocalyptic number.

66801991 is a deficient number, since it is larger than the sum of its proper divisors (68001).

66801991 is an equidigital number, since it uses as much as digits as its factorization.

66801991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 68000.

The product of its (nonzero) digits is 23328, while the sum is 40.

The square root of 66801991 is about 8173.2484973846. The cubic root of 66801991 is about 405.7543042551.

The spelling of 66801991 in words is "sixty-six million, eight hundred one thousand, nine hundred ninety-one".