How to solve this:

Find The value of :: \(\displaystyle{\sum _{ 0 }^{ n }{ { { ^{ n }{ P } } }_{ r } }} \).

here P(n,r) means permutation of n things taken r at a time .

I have not study Binaomial chapter in detaied Yet . But I need this sumition whlile solving an question of PNC .

PLease Help , I have no idea for such summitions , Please Explain , Thanks a lot

## Comments

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TopNewestWe can actually evaluate the sum as \(\Gamma(n+1,1)\) , where \(\Gamma(a,x)\) is the Upper Incomplete Gamma function .

\[\begin{align} \sum_{r=0}^{n} \binom{n}{r} r! &= \sum_{r=0}^{n} \binom{n}{r} \int_{0}^{\infty} e^{-x} x^r \mathrm{d}x\\ &= \int_{0}^{\infty} \sum_{r=0}^{n} \binom{n}{r} e^{-x} x^r \mathrm{d}x\\ &= \int_{0}^{\infty} e^{-x} (1+x)^{n} \mathrm{d}x\\ &=\boxed{e \cdot \Gamma(n+1,1)}\end{align}\] .

This solution has been inspired from the genius Pratik Shastri's solution to this problem . – Azhaghu Roopesh M · 2 years, 5 months ago

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– Megh Choksi · 2 years, 5 months ago

Do you know how to add a question in a white box?Log in to reply

– Pratik Shastri · 2 years, 5 months ago

Add ">>" before the text.Log in to reply

– Azhaghu Roopesh M · 2 years, 5 months ago

Use \boxed{}Log in to reply

– Megh Choksi · 2 years, 5 months ago

No , I am asking the way Karan posted the question in a white box , I am toogling latex but it does'nt show the code for itLog in to reply

@KARAN SHEKHAWAT himself . – Azhaghu Roopesh M · 2 years, 5 months ago

Nope. Let's askLog in to reply

– Karan Shekhawat · 2 years, 5 months ago

Ohh , so it is gamma summition . Can we use any other approach ? Since I didn't Know anything about gamma function , or can you please tell something about this gamma function ? I'am nwe to it . ThanksLog in to reply

– Azhaghu Roopesh M · 2 years, 5 months ago

Well even I am new to Gamma function . If you study the topic from Wolfram Mathworld , you will get some understanding . It's actually quite easy , we are just exploiting the properties of Gamma Function .Log in to reply

– Megh Choksi · 2 years, 5 months ago

Can you please explain the second step , not gettingLog in to reply

\[\begin{align} \int_{0}^{\infty} \sum_{r=0}^{n} \binom{n}{r} e^{-x} x^r \mathrm{d}x\\ \end{align}\] and we all know that \[ \sum_{r=0}^{n} \binom{n}{r} x^r = (1+x)^{n} \] . And after that we just use the definition of Incomplete Upper Gamma Function . – Azhaghu Roopesh M · 2 years, 5 months ago

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howdid you take the summation into integral , how is this valid , can you explain in more detail please – Megh Choksi · 2 years, 5 months agoLog in to reply

x[i.e.dx] and the summation is independant ofx[it's dependant onr]so I don't think that we should have a problem taking it into the integral . – Azhaghu Roopesh M · 2 years, 5 months agoLog in to reply

– Megh Choksi · 2 years, 5 months ago

their upper limits are different , then also can we slide the summation into integral (how?) any reason?Log in to reply

this out. – Pratik Shastri · 2 years, 5 months ago

CheckLog in to reply

– Azhaghu Roopesh M · 2 years, 5 months ago

Since their limits are different, we can almost treat the Summation Term as a constant w.r.t the Integral (That's what I think) . BTW I'll ask Pratik Shastri and let you knowLog in to reply