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Collecting multiple function arguments into a List?


I have a problem with 8 input variables, call 'em  x1, x2, ... , x8, and 
I want to define a bunch of functions (using :=, not =) that depend on 
various subsets of these variables, e.g.

   f1[ x1_, x2_, x4_] :=  << stuff involving x1, x2, x4 >>
   
   f2[ x1_, x3_] :=  << stuff involving  x1 and x3 only >> 
   
   f3[[x2_, x3_, x5_, x8_} := << stuff involving x2, x3, x5, x8 >>

To minimize typing and bookkeeping I'd like to define a list of the 
input variables, e.g.

   vars = {x1, x2, x3, x4, x5, x6, x7, x8}
   
and then write all the function definitions in the simplified syntax

   f1[vars_] := << same stuff as above >>
   f2[vars_] := << same stuff as above >>
   f3[vars_] := << same stuff as above >>
   
Can I do this?  What's the required syntax?

   vars = {x1, x2, x3 ...} ??   OR   vars = {x1_, x2_, x3_} ??
   
   fn[vars] :=  << >> ??   OR   fn[vars_] := << >>??
   
Thanks,  siegman at stanford.edu


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