The double sequence ( y ( n , m ) ) n = 0 , m = 0 ∞ is defined by the following recursive equations, y ( 0 , 0 ) = e 6 y ( 0 , m ) = 3 y ( 0 , m − 1 ) , m ≥ 1 y ( n , 0 ) = 2 y ( n − 1 , 0 ) , n ≥ 1 y ( n , m ) = m y ( n − 1 , m − 1 ) + 2 y ( n − 1 , m ) , n ≥ 1 , m ≥ 1 .
Then if A = ∑ i = 0 1 0 0 ( i 1 0 0 ) ( − 1 ) i 2 i y ( 1 0 0 − i , 1 0 0 ) , find ⌊ ln A ⌋ .
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How did you find the function ?
I found a pattern and an expression for y(m,n) in terms of y(0,i) where i is from n-m to n, then A becomes a sum inside sum. but I couldn't evaluate that nested sum
I understand Paramananda Das. Eventually, you might obtain also a pattern for the sum and get the answer. But then the most challenging part would be to prove that formula by induction.
To find the function requires certain experience with this kind of problems and to do a lot of trials.
But now that you seem to understand how to solve this problem by the introduction of a function, this becomes another tool that you can use in the future to solve similar problems.
super easy solution very interesting one too
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The solution to this problem will be simpler if we consider the following double sequence of functions : f ( x , n , m ) = D n ( x m e 2 x ) , where the D n represents the n -th derivative of the given function. It is easy to check that f ( 3 , n , m ) satisfies all the recursive equations given in the problem and that f ( 3 , 0 , 0 ) = e 6 . Then, it can be proved by mathematical induction that f ( 3 , n , m ) = y ( n , m )
Now, we can prove that if p ( t ) = a n t n + . . . + a 0 , then
i = 0 ∑ n a i f ( x , i , n ) = i = 0 ∑ n a i D i ( x n e 2 x ) = i = 0 ∑ n a i l = 0 ∑ i ( l i ) ( n − l ) ! n ! x n − l 2 i − l e 2 x = i = 0 ∑ n a i l = 0 ∑ i ( l n ) ( i − l ) ! ( i ) ! x n − l 2 i − l e 2 x = = l = 0 ∑ n ( l n ) x n − l i = l ∑ n a i ( i − l ) ! ( i ) ! 2 i − l e 2 x = l = 0 ∑ n ( l n ) x n − l p ( l ) ( 2 ) e 2 x Let us consider the polynomial p ( t ) = ( t − 2 ) 1 0 0 = ∑ i = 0 1 0 0 ( i 1 0 0 ) ( − 1 ) i t 1 0 0 − i 2 i . Therefore, i = 0 ∑ 1 0 0 ( i 1 0 0 ) ( − 1 ) i 2 i . f ( x , 1 0 0 − i , 1 0 0 ) = l = 0 ∑ 1 0 0 ( l 1 0 0 ) x 1 0 0 − l p ( l ) ( 2 ) e 2 x Now, it is easy to see that p ( i ) ( 2 ) = 0 for all i such that 0 ≤ i ≤ 9 9 . But p ( 1 0 0 ) ( 2 ) = 1 0 0 ! . So the value of previous sum is 1 0 0 ! e 2 x . But in the case that x = 3 , that sum is 1 0 0 ! e 6 . So A = 1 0 0 ! e 6 , then ln A = 3 6 9 . 7 . . . , and, therefore, the answer of this problem is 3 6 9 .