391663721 has 2 divisors, whose sum is σ = 391663722. Its totient is φ = 391663720.

The previous prime is 391663691. The next prime is 391663729. The reversal of 391663721 is 127366193.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 303979225 + 87684496 = 17435^2 + 9364^2 .

It is a cyclic number.

It is not a de Polignac number, because 391663721 - 2¹⁰ = 391662697 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (391663729) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 195831860 + 195831861.

It is an arithmetic number, because the mean of its divisors is an integer number (195831861).

Almost surely, 2^391663721 is an apocalyptic number.

It is an amenable number.

391663721 is a deficient number, since it is larger than the sum of its proper divisors (1).

391663721 is an equidigital number, since it uses as much as digits as its factorization.

391663721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 40824, while the sum is 38.

The square root of 391663721 is about 19790.4957239580. The cubic root of 391663721 is about 731.6518053193.

The spelling of 391663721 in words is "three hundred ninety-one million, six hundred sixty-three thousand, seven hundred twenty-one".