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"Difference Equations" by Walter G. Kelley and Allan C. Peterson, 2nd Edition, gives an example on how to approximate $\sum_{k=1}^n{k^{1/2}}$ using integrals and Bernoulli numbers.

I'm interested in nesting summations and using integrals to approximate them. So I cooked up a relatively simple example:

$\sum_{j=0}^n{\sum_{k=0}^j{c^j k^{1/2}}}$

I'm mainly interested in knowing how to include an estimate from nested integrals. The book gives the Euler summation formula:

$\displaystyle\sum_{k=1}^n{f(k)} = $

$\displaystyle\int_1^n{f(t)dt}+\frac{f(n)+f(1)}{2} + $ $\displaystyle\sum_{i=1}^m{\frac{B_{2i}}{(2i)!}\left(f^{(2i-1)}(n)-f^{(2i-1)}(1)\right)} - $ $\displaystyle\frac{1}{(2m)!}\int_1^n{f^{(2m)}(t)B_{2m}(t-\lfloor t \rfloor)dt}$

where $B_i$ represents the $i$th Bernoulli number. This formula allows one to estimate a summation by using integrals and Bernoulli numbers. More information can also be found here, in Wikipedia's entry on it.

I'm gussing what I can do is start with $\sum_j \sum_k f(j,k)$ and plug in $\sum_k f(j,k)$ into the Euler summation formula to get half of a big formula. Then plug that in, along with $\sum_j$, into a second Euler summation formula.

QUESTION

Can someone please show me how I can approximate the solutions by using Euler summations? It would help me a great deal, so I'd be very greatful! Thanks for reading.

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Your sum is $\sum_{j=0}^nc^j\sum_{k=0}^j\sqrt k$ Euler summation on the inner sum should give you something like $\sum_{k=0}^j\sqrt k=(2/3)j^{3/2}+{\rm\ lower\ terms}$ So now you're after $\sum_{j=0}^nj^{3/2}c^j$ Euler summation will compare this to $\int_0^nt^{3/2}c^t\,dt$ which looks to me like an "incomplete Gamma function," q.v.

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    Then I guess you'll have to carry out the Euler summation in more detail.2011-11-07