66521891 has 4 divisors (see below), whose sum is σ = 66546552. Its totient is φ = 66497232.

The previous prime is 66521887. The next prime is 66521909. The reversal of 66521891 is 19812566.

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

It is a cyclic number.

It is not a de Polignac number, because 66521891 - 2² = 66521887 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (66521291) 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, 7706 + ... + 13871.

It is an arithmetic number, because the mean of its divisors is an integer number (16636638).

Almost surely, 2^66521891 is an apocalyptic number.

66521891 is a deficient number, since it is larger than the sum of its proper divisors (24661).

66521891 is a wasteful number, since it uses less digits than its factorization.

66521891 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 24660.

The product of its digits is 25920, while the sum is 38.

The square root of 66521891 is about 8156.0953280353. The cubic root of 66521891 is about 405.1864020271.

The spelling of 66521891 in words is "sixty-six million, five hundred twenty-one thousand, eight hundred ninety-one".