66931721 has 4 divisors (see below), whose sum is σ = 68564244. Its totient is φ = 65299200.

The previous prime is 66931687. The next prime is 66931741. The reversal of 66931721 is 12713966.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12713966 = 2 ⋅6356983.

It can be written as a sum of positive squares in 2 ways, for example, as 2755600 + 64176121 = 1660^2 + 8011^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-66931721 is a prime.

It is a Duffinian number.

It is a zygodrome in base 3.

It is not an unprimeable number, because it can be changed into a prime (66931741) 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, 816200 + ... + 816281.

It is an arithmetic number, because the mean of its divisors is an integer number (17141061).

Almost surely, 2^66931721 is an apocalyptic number.

It is an amenable number.

66931721 is a deficient number, since it is larger than the sum of its proper divisors (1632523).

66931721 is a wasteful number, since it uses less digits than its factorization.

66931721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1632522.

The product of its digits is 13608, while the sum is 35.

The square root of 66931721 is about 8181.1809049794. The cubic root of 66931721 is about 406.0167937758.

The spelling of 66931721 in words is "sixty-six million, nine hundred thirty-one thousand, seven hundred twenty-one".