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Correct answer: 204
We need to count the number of matrices with each entry in such that the sum of all four entries is a prime number in .
1. Possible prime sums
Primes in the interval are So we must count the number of ordered quadruples with total sum equal to or .
Thus the problem reduces to finding the coefficient of in
We will count each sum separately.
2. Count solutions for sum
We need the number of nonnegative integer solutions of with each .
Since the sum is only , the upper bound is automatically satisfied.
Number of solutions:
So,
3. Count solutions for sum
We need the number of solutions of
First count all nonnegative solutions:
Now subtract those where some variable is at least .
If , write . Then which has exactly solution. Similarly for each of the 4 variables.
No two variables can both be at least because the total sum is .
Hence
4. Count solutions for sum
We need the number of solutions of
Total nonnegative solutions:
Subtract solutions with some .
For a fixed variable, say , let . Then Number of solutions: There are 4 choices of variable, so subtract
Now check intersections: if two variables are each at least , their sum is at least , impossible. So no intersections occur.
Thus
5. Count solutions for sum
We need the number of solutions of
Total nonnegative solutions:
Subtract solutions with at least one variable
For a fixed variable, say , let . Then Number of solutions: With 4 choices:
Add back intersections of two variables each
Suppose and . Put Then Number of solutions: Number of pairs of variables: So add back
Triple intersections
If three variables are each at least , the sum is at least , impossible. So no further terms.
Hence
6. Total count
Therefore the required number of matrices is
7. Comparison with stored answer
Our derived answer is which matches the stored correct answer.
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