66721815 has 12 divisors (see below), whose sum is σ = 115651224. Its totient is φ = 35584944.

The previous prime is 66721799. The next prime is 66721819. The reversal of 66721815 is 51812766.

It is not a de Polignac number, because 66721815 - 2⁴ = 66721799 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 66721815.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (66721819) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 741309 + ... + 741398.

It is an arithmetic number, because the mean of its divisors is an integer number (9637602).

Almost surely, 2^66721815 is an apocalyptic number.

66721815 is a deficient number, since it is larger than the sum of its proper divisors (48929409).

66721815 is a wasteful number, since it uses less digits than its factorization.

66721815 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1482718 (or 1482715 counting only the distinct ones).

The product of its digits is 20160, while the sum is 36.

The square root of 66721815 is about 8168.3422430748. The cubic root of 66721815 is about 405.5919099900.

The spelling of 66721815 in words is "sixty-six million, seven hundred twenty-one thousand, eight hundred fifteen".