Seperating a Summation problem.

[DKATyler]

[SOLVED] Seperating a Summation problem.

Homework Statement

The Problem:
Separate a sum into 2 pieces (part of a proof problem).

Using: [tex]X=
\sum^{n}_{k=1}\frac{n!}{(n-k)!}
[/tex]

Solve in relation to n and X:
[tex]
\sum^{n+1}_{k=1}\frac{(n+1)!}{(n+1-k)!}
[/tex]

Homework Equations

?

The Attempt at a Solution

[tex]
\sum^{n}_{k=1}[\frac{(n+1)!}{(n+1-k)!}]+\frac{(n+1)!}{(n+1-[n+1])!}
[/tex]


[tex]
\sum^{n}_{k=1}[\frac{(n)!}{(n-k)!}*\frac{(n+1)}{(n+1-k)}]+\frac{(n+1)!}{(n+1-[n+1])!}
[/tex]


[tex]
(n+1)*\sum^{n}_{k=1}[\frac{(n)!}{(n-k)!}*\frac{1}{(n+1-k)}]+(n+1)!}
[/tex]


I think this is fairly close but, I have no way of getting rid of the 1/(n+1-k) term.

 

[Defennder]

Can you show that [tex]\sum_{k=2}^n \frac{n!}{(n-k+1)!} \ + \ \frac{n!}{(n+1-(n+1))!} = \sum^{n}_{k=1}\frac{n!}{(n-k)!} [/tex]?

If you do that, you can express [tex]\sum^{n+1}_{k=1}\frac{(n+1)!}{(n+1-k)!}[/tex] as a summation starting from k=2. Then you should be able to get the desired expression.

Try it out for some values of k and n, then you'll see a pattern.

In general the pattern is [tex]\sum_{k=1}^{n}f(k) = \sum_{k=2}^{n} f(k-1)\ +\ f(n)[/tex]

 

[DKATyler]

That helps quite a bit.

[tex]
n!+\sum^{n}_{k=2}\frac{n!}{(n+1-k)!}=\sum^{n}_{k=1}\frac{n!}{(n-k)!}=X
[/tex]

So continuing from this step:
[tex]
(n+1)*\sum^{n}_{k=1}\frac{n!}{(n-k+1)!}+\frac{(n+1)!}{(n+1-[n+1])!}
[/tex]

changing the Index and adding/subtracting n!
[tex]
(n+1)*(1-n!+n!+\sum^{n}_{k=2}\frac{n!}{(n-k+1)!})+(n+1)!
[/tex]

Solves the Equation in terms of n and X:
[tex]
(n+1)*(1-n!+X)+(n+1)!
[/tex]

Yep, that worked, now I can complete the rest of the proof :) Thank you very much. How to mark this "[solved]?"

 

[Defennder]

Go to your first post in this thread, at the top right hand corner of the post marked "Thread Tools"