# Bit-Sum Prime

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Bit-Sum Prime

Diﬃculty level: moderate

Every student, who has learned programming, must have written a program to determine whether a given positive integer is a prime number. Basically in order to determine whether a positive integer n is prime, we search for any number in the range [2, n − 1] which can divide n. Some of you would have designed a slighly better implementation where you search √ for any divisor of n from the range [2, n ]. Does either of these two implementations correspond to an eﬃcient algorithm ? Ponder over this question deeply. Let us try to design an eﬃcient algorithm for a problem which looks as simple and innocent as the problem discussed above. First, we give a deﬁnition. Deﬁnition 1. A positive integer is said to be bit-sum prime if the sum of the bits in its binary representation is a prime number. For example 6(110), 14(1110) are bit-sum prime numbers, whereas 29(11101) is not a bit-sum prime number. Design and implement an algorithm which receives a 64-bit integer n and outputs the count of all the bit-sum prime numbers less than n. Test it for a really large number, for example, execute your algorithm for 123456789123456789. Hint: Your algorithm/program is NOT supposed to enumerate all the bit-sum prime numbers. Instead, it has to just report the count of all the bit-sum prime number. Hence the output will be just a single number. Notice that you will have to use clever programming skills also in this problem.

every art is beautiful and so is the art of algorithm design ...

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