66540721 has 4 divisors (see below), whose sum is σ = 71659252. Its totient is φ = 61422192.

The previous prime is 66540709. The next prime is 66540743. The reversal of 66540721 is 12704566.

It is a happy number.

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

It can be written as a sum of positive squares in 2 ways, for example, as 39125025 + 27415696 = 6255^2 + 5236^2 .

It is not a de Polignac number, because 66540721 - 2⁹ = 66540209 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (66540751) 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, 2559246 + ... + 2559271.

It is an arithmetic number, because the mean of its divisors is an integer number (17914813).

Almost surely, 2^66540721 is an apocalyptic number.

It is an amenable number.

66540721 is a deficient number, since it is larger than the sum of its proper divisors (5118531).

66540721 is a wasteful number, since it uses less digits than its factorization.

66540721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5118530.

The product of its (nonzero) digits is 10080, while the sum is 31.

The square root of 66540721 is about 8157.2495977505. The cubic root of 66540721 is about 405.2246297446.

The spelling of 66540721 in words is "sixty-six million, five hundred forty thousand, seven hundred twenty-one".