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Suppose we have a number n, we have to check whether n can be expressed as a sum of two semi-primes or not.

As we know the semi-prime is a number if it can be expressed as product of two primes number. First few semi-prime numbers are (1 - 100 range): 4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 62, 65, 69, 74, 77, 82, 85, 86, 87, 91, 93, 94, 95.

So, if the input is like n = 108, then the output will be True as this is sum of 14 and 94 both are semi-prime.

To solve this, we will follow these steps −

- MAX := 10000 assuming given inputs are sum of semi-primes which are in range 1 to 10000
- nums := a new list
- s_prime_flags := an array of size MAX and fill with False
- Define a function get_semi_primes() . This will take
- for i in range 2 to MAX - 1, do
- count := 0
- num := i
- j := 2
- while count < 2 and j^2 <= num, do
- while num is divisible by j, do
- num := num / j
- count := count + 1

- j := j + 1

- while num is divisible by j, do
- if num > 1, then
- count := count + 1

- if count is same as 2, then
- s_prime_flags[i] := True

- insert i at the end of nums
- From the main method do the following −
- call get_semi_primes()
- i := 0
- while nums[i] <= quotient of (n / 2), do
- if s_prime_flags[n - nums[i]] is True, then
- return True

- i := i + 1

- if s_prime_flags[n - nums[i]] is True, then
- return False

Let us see the following implementation to get better understanding −

MAX = 10000 nums = [] s_prime_flags = [False] * MAX def get_semi_primes(): for i in range(2, MAX): count = 0 num = i j = 2 while count < 2 and j * j <= num: while num % j == 0: num /= j count += 1 j += 1 if num > 1: count += 1 if count == 2: s_prime_flags[i] = True nums.append(i) def solve(n): get_semi_primes() i = 0 while nums[i] <= n // 2: if s_prime_flags[n - nums[i]] == True: return True i += 1 return False n = 108 print(solve(n))

[4, 2, 3], 11

True

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