Re: alternating sums
- To: mathgroup at christensen.cybernetics.net
- Subject: [mg321] Re: [mg307] alternating sums
- From: Richard Mercer <richard at rmercer.wright.edu>
- Date: Mon, 12 Dec 1994 15:11:48 -0500
> Here's an elementary question that has me partially
> stumped. I often have occasion to convert mylist =
> {a1,a2,a3,...} into an alternating sum, a1-a2+a3-a4+...
> . I find it aesthetically unsatisfactory to generate a
> list of alternating plus and minus ones, multiply mylist
> with the alternating list and then Apply Plus to the
> product. It works of course, but I have a feeling there
> is a much better way to do this. (This is clearly not
> of earthshaking importance!!. I'm just curious.) Jack
> Goldberg Univ of Mich Mathematics
Jack,
Here's the best I could do off the top of my head. I suspect there's
something simpler still, but...
In[49]:=
mylist = {a1,a2,a3,a4,a5,a6};
Plus @@ Apply[Subtract,Partition[mylist,2],{1}]
Out[50]=
a1 - a2 + a3 - a4 + a5 - a6
Richard Mercer