638657561 has 2 divisors, whose sum is σ = 638657562. Its totient is φ = 638657560.

The previous prime is 638657557. The next prime is 638657563. The reversal of 638657561 is 165756836.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 518973961 + 119683600 = 22781^2 + 10940^2 .

It is a cyclic number.

It is not a de Polignac number, because 638657561 - 2² = 638657557 is a prime.

Together with 638657563, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (638657563) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 319328780 + 319328781.

It is an arithmetic number, because the mean of its divisors is an integer number (319328781).

Almost surely, 2^638657561 is an apocalyptic number.

It is an amenable number.

638657561 is a deficient number, since it is larger than the sum of its proper divisors (1).

638657561 is an equidigital number, since it uses as much as digits as its factorization.

638657561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 907200, while the sum is 47.

The square root of 638657561 is about 25271.6750730932. The cubic root of 638657561 is about 861.1709131592.

The spelling of 638657561 in words is "six hundred thirty-eight million, six hundred fifty-seven thousand, five hundred sixty-one".