66156721 has 2 divisors, whose sum is σ = 66156722. Its totient is φ = 66156720.

The previous prime is 66156719. The next prime is 66156731. The reversal of 66156721 is 12765166.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 62236321 + 3920400 = 7889^2 + 1980^2 .

It is a cyclic number.

It is not a de Polignac number, because 66156721 - 2¹ = 66156719 is a prime.

Together with 66156719, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (66156731) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 33078360 + 33078361.

It is an arithmetic number, because the mean of its divisors is an integer number (33078361).

Almost surely, 2^66156721 is an apocalyptic number.

It is an amenable number.

66156721 is a deficient number, since it is larger than the sum of its proper divisors (1).

66156721 is an equidigital number, since it uses as much as digits as its factorization.

66156721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 15120, while the sum is 34.

The square root of 66156721 is about 8133.6781962406. The cubic root of 66156721 is about 404.4436215221.

The spelling of 66156721 in words is "sixty-six million, one hundred fifty-six thousand, seven hundred twenty-one".