638721791 has 2 divisors, whose sum is σ = 638721792. Its totient is φ = 638721790.

The previous prime is 638721779. The next prime is 638721833. The reversal of 638721791 is 197127836.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 638721791 - 2²⁶ = 571612927 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (638721701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 319360895 + 319360896.

It is an arithmetic number, because the mean of its divisors is an integer number (319360896).

Almost surely, 2^638721791 is an apocalyptic number.

638721791 is a deficient number, since it is larger than the sum of its proper divisors (1).

638721791 is an equidigital number, since it uses as much as digits as its factorization.

638721791 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 127008, while the sum is 44.

The square root of 638721791 is about 25272.9458314618. The cubic root of 638721791 is about 861.1997816050.

The spelling of 638721791 in words is "six hundred thirty-eight million, seven hundred twenty-one thousand, seven hundred ninety-one".