Recursion for a Double Sequence

Calculus Level 5

The double sequence ( y ( n , m ) ) n = 0 , m = 0 (y(n,m))_{n=0, m=0}^{\infty} is defined by the following recursive equations, y ( 0 , 0 ) = e 6 y(0, 0)=e^6 y ( 0 , m ) = 3 y ( 0 , m 1 ) , m 1 y(0, m)=3y(0, m-1), \quad \quad m\geq 1 y ( n , 0 ) = 2 y ( n 1 , 0 ) , n 1 y(n,0)=2y(n-1, 0),\quad \quad n\geq 1 y ( n , m ) = m y ( n 1 , m 1 ) + 2 y ( n 1 , m ) , n 1 , m 1. y(n, m)= m y(n-1, m-1)+2 y(n-1, m),\quad \quad n\geq 1, m\geq 1.

Then if A = i = 0 100 ( 100 i ) ( 1 ) i 2 i y ( 100 i , 100 ) , A=\sum_{i=0}^{100} \binom{100}{i}(-1)^i 2^iy(100-i, 100), find ln A . \lfloor \ln A \rfloor.


The answer is 369.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Arturo Presa
Jun 12, 2020

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 ) , f(x, n, m)=D^n(x^m e^{2x}), where the D n D^n represents the n n -th derivative of the given function. It is easy to check that f ( 3 , n , m ) f(3, n, m) satisfies all the recursive equations given in the problem and that f ( 3 , 0 , 0 ) = e 6 . f(3, 0, 0)=e^6. Then, it can be proved by mathematical induction that f ( 3 , n , m ) = y ( n , m ) f(3, n, m)=y(n, m)

Now, we can prove that if p ( t ) = a n t n + . . . + a 0 , 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 ( i l ) n ! ( n l ) ! x n l 2 i l e 2 x = i = 0 n a i l = 0 i ( n l ) ( i ) ! ( i l ) ! x n l 2 i l e 2 x = \sum_{i=0}^n a_i f(x, i, n)=\sum_{i=0}^n a_i D^{i}(x^ne^{2x})=\sum_{i=0}^n a_i \sum_{l=0}^{i} \binom{i}{l} \frac{n!}{(n-l)!}x^{n-l}2^{i-l} e^{2x}=\sum_{i=0}^n a_i \sum_{l=0}^{i} \binom{n}{l} \frac{(i)!}{(i-l)!}x^{n-l}2^{i-l} e^{2x} = = l = 0 n ( n l ) x n l i = l n a i ( i ) ! ( i l ) ! 2 i l e 2 x = l = 0 n ( n l ) x n l p ( l ) ( 2 ) e 2 x =\sum_{l=0}^n\binom{n}{l} x^{n-l} \sum_{i=l}^{n} a_i \frac{(i)!}{(i-l)!}2^{i-l}e^{2x}=\sum_{l=0}^n\binom{n}{l} x^{n-l} p^{(l)}(2)e^{2x} Let us consider the polynomial p ( t ) = ( t 2 ) 100 = i = 0 100 ( 100 i ) ( 1 ) i t 100 i 2 i . p(t)=(t-2)^{100}=\sum_{i=0}^{100}\binom{100}{i}(-1)^it^{100-i} 2^i. Therefore, i = 0 100 ( 100 i ) ( 1 ) i 2 i . f ( x , 100 i , 100 ) = l = 0 100 ( 100 l ) x 100 l p ( l ) ( 2 ) e 2 x \sum_{i=0}^{100} \binom{100}{i}(-1)^i 2^i.f(x,100- i, 100)=\sum_{l=0}^{100}\binom{100}{l} x^{100-l} p^{(l)}(2)e^{2x} Now, it is easy to see that p ( i ) ( 2 ) = 0 p^{(i)}(2)=0 for all i i such that 0 i 99. 0\leq i\leq 99. But p ( 100 ) ( 2 ) = 100 ! . p^{(100)}(2)=100!. So the value of previous sum is 100 ! e 2 x . 100!e^{2x}. But in the case that x = 3 , x=3, that sum is 100 ! e 6 . 100! e^6. So A = 100 ! e 6 , A=100!e^6, then ln A = 369.7... , \ln A=369.7..., and, therefore, the answer of this problem is 369. \boxed{369.}

How did you find the function ?

Paramananda Das - 12 months ago

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

Paramananda Das - 12 months ago

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.

Arturo Presa - 12 months ago

To find the function requires certain experience with this kind of problems and to do a lot of trials.

Arturo Presa - 12 months ago

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.

Arturo Presa - 12 months ago

super easy solution very interesting one too

Aditya Ekbote - 7 months, 1 week ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...