Re: Summation question

*To*: mathgroup at smc.vnet.net*Subject*: [mg85709] Re: Summation question*From*: Albert Retey <awnl at arcor.net>*Date*: Tue, 19 Feb 2008 07:08:54 -0500 (EST)*References*: <fpdu7v$qlq$1@smc.vnet.net>

Hi, > I want to make a summation like this: Sum[f[i],{i, start, > finish}]. What I want is that the sum be over a specific set like > {1,3,5,7,11,13}. So the sum would be : f[1]+f[3]+f[5]+f[7]+f[11]+f[13]. The > set I want to use is the primes. So, how do I use, if possible, Sum over a > specific set ? > In version 6 you can use a list of values as iterator in many functions, among them Table and Sum: In[23]:= Sum[i,{i,{a,b,c}}] Out[23]= a+b+c So what remains is to construct a list of the values for which you want to sum, the following is a very simple way to extract the primes from 20 to 30. If you will work with very large ranges, you might need something smarter here: In[28]:= Select[Range[20,30],PrimeQ] Out[28]= {23,29} In combination this will do what you need: In[27]:= Sum[f[i],{i,Select[Range[5,20],PrimeQ]}] Out[27]= f[5]+f[7]+f[11]+f[13]+f[17]+f[19] hth, albert