• 641 can be written using four 4's:

641 has 2 divisors, whose sum is σ = 642. Its totient is φ = 640.

The previous prime is 631. The next prime is 643. The reversal of 641 is 146.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 625 + 16 = 25^2 + 4^2 .

It is a sliding number, since 641 = 16 + 625 and 1/16 + 1/625 = 0.0641.

It is a cyclic number.

It is not a de Polignac number, because 641 - 2^{6} = 577 is a prime.

It is a Sophie Germain prime.

Together with 643, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a plaindrome in base 12 and base 14.

It is a nialpdrome in base 10 and base 11.

It is not a weakly prime, because it can be changed into another prime (643) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is a good prime.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 320 + 321.

It is an arithmetic number, because the mean of its divisors is an integer number (321).

It is a Proth number, since it is equal to 5 ⋅ 2^{7} + 1 and 5 < 2^{7}.

It is an amenable number.

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

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

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

The product of its digits is 24, while the sum is 11.

The square root of 641 is about 25.3179778023. The cubic root of 641 is about 8.6222248300.

Subtracting from 641 its sum of digits (11), we obtain a triangular number (630 = T_{35}).

Adding to 641 its reverse (146), we get a palindrome (787).

The spelling of 641 in words is "six hundred forty-one", and thus it is an aban number.

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