# Hyperfactorial function

• Nov 5th 2009, 03:01 AM
NonCommAlg
Hyperfactorial function
Prove that $\forall n \in \mathbb{N}: \ \ \prod_{k=1}^n k^k < (n+1)^{\frac{n(n+1)}{2}} e^{-\frac{(n-1)^2}{4}}.$
• Nov 10th 2009, 12:05 AM
simplependulum
Could we prove it by induction ? I think you must have a great method to solve it ( without induction )
• Nov 10th 2009, 01:57 AM
NonCommAlg
Quote:

Originally Posted by simplependulum
Could we prove it by induction ? I think you must have a great method to solve it ( without induction )

induction would be fine if you could finish it. another way is to use Euler's summation formula to find an upper bound for $\sum_{k=1}^n k \ln k.$ (Wink)
• Nov 15th 2009, 08:55 AM
girdav
Using logarithm, we have to show that $\frac{\left(n-1\right)^2}4\leq \sum_{k=1}^n k\left(\ln \left(n+1\right)-\ln \left(k\right)\right)$.
We have $\sum_{k=1}^n k\left(\ln \left(n+1\right)-\ln \left(k\right)\right) = \sum_{k=1}^n k\ln \left( 1+\frac{n+1}k-1\right)$ and $\frac{n+1}k-1\geq 0$ so using $\ln\left(1+x\right) \geq x-\frac{x^2}2$ we should have the result.