66661721 has 4 divisors (see below), whose sum is σ = 76184832. Its totient is φ = 57138612.

The previous prime is 66661669. The next prime is 66661733. The reversal of 66661721 is 12716666.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 12716666 = 2 ⋅6358333.

It is a cyclic number.

It is not a de Polignac number, because 66661721 - 2⁶ = 66661657 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (66661741) 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, 4761545 + ... + 4761558.

It is an arithmetic number, because the mean of its divisors is an integer number (19046208).

Almost surely, 2^66661721 is an apocalyptic number.

It is an amenable number.

66661721 is a deficient number, since it is larger than the sum of its proper divisors (9523111).

66661721 is an equidigital number, since it uses as much as digits as its factorization.

66661721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9523110.

The product of its digits is 18144, while the sum is 35.

The square root of 66661721 is about 8164.6629446659. The cubic root of 66661721 is about 405.4701059424.

The spelling of 66661721 in words is "sixty-six million, six hundred sixty-one thousand, seven hundred twenty-one".