MathGroup Archive 2010

[Date Index] [Thread Index] [Author Index]

Search the Archive

Simplifying to PolyLog


Hi,

I'm puzzled by the fact that the first of the following expressions is 
successfully summed while the second, simpler one is not:

In[806]:= Sum[(((-1)^n) (r^n) Cos[a n] )/n^3, {n, 1, Infinity}]
Sum[(((-1)^n) Cos[a n] )/n^3, {n, 1, Infinity}]

Out[806]= 1/2 (PolyLog[3, -E^(-I a) r] + PolyLog[3, -E^(I a) r])

Out[807]= \!\(
\*UnderoverscriptBox[\(\[Sum]\), \(n = 1\), \(\[Infinity]\)]
\*FractionBox[\(
\*SuperscriptBox[\((\(-1\))\), \(n\)]\ Cos[a\ n]\),
SuperscriptBox[\(n\), \(3\)]]\)

Many thanks in advance,

Sam


  • Prev by Date: Puzzled by a puzzle
  • Next by Date: How does Plot* functions know about range?
  • Previous by thread: Re: Puzzled by a puzzle
  • Next by thread: How does Plot* functions know about range?