Consider the following number .
Suppose is the number of non-zero elements of row of Pascal's triangle in modulo . (To make this clear, row would be the row with elements) What is ?
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Well, this question looks difficult(at least to me) at first sight but it is very easy(including that easy guess).
Well, we will not first solve the sum.
In Pascal's triangle, rows consist of numbers of the form C ( n − 1 , k ) for row n and for k = [ 0 , n − 1 ] .
In ( S + 1 ) th row, the n − 1 would be just equal to S .
Next, we have to find C ( S , k ) m o d 3 7 for k = [ 0 , S ] (hence S + 1 elements)
Now, Lucas Theorem says C ( n , r ) m o d p is the product of combination operators of the corresponding coefficients of the base p expansion of n and r .
S = 9 ∗ 3 7 3 7 + 9 ∗ 3 7 3 6 + . . . . . + 9 ∗ 3 7 + 0 ∗ 3 7
and k = n 3 7 p 3 7 + n 3 6 p 3 6 + . . . . + n 1 p + n 0
Hence, C ( S , k ) = C ( 9 , n 3 7 ) ∗ C ( 9 , n 3 6 ) ∗ C ( 9 , n 3 5 ) . . . . C ( 9 , n 1 ) ∗ C ( 0 , n 0 ) m o d 3 7
Now the remainder side will be non-zero if n k ≤ 9 (except for n 0 when it has to be equal to 0 only because if it is greater than 0 , the operator becomes 0 and i f it is less than 0 , it again becomes 0 . So,
n k = [ 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ] (for k = [ 1 , 3 7 ] )
and n 0 = 0
As a result, we can use Law of Multiplication of counting and get the final answer as - 1 0 3 7 which will be followed by l o g 1 0 3 7 = 3 7