MapAt complaint

*To*: mathgroup at yoda.physics.unc.edu*Subject*: MapAt complaint*From*: wmm at chem.wayne.edu (Martin McClain)*Date*: Fri, 5 Nov 93 14:46:43 EST

Dear MathGroupers: I am a fairly experienced Mma user, but quite frequently I cannot get MapAt to do what I want. My question: Am I missing something obvious, or does MapAt really need to be extended? Here is the latest example: I want to go from expr1 = 1+Sin[a+b]+Sin[a-b] to expr2 = 1 + 2 Cos[a] Sin[b]. Simplify or Factor[#,Trig->True]& does this if applied to parts 2 and 3 of expr1, but it gets discouraged and does nothing if applied to the whole thing. So alternatively, I would like to use MapAt to make Simplify work on parts 2 and 3 together. But I can't find any one-line way to do this. If you try MapAt[Simplify,expr1,{{2},{3}}] it seems to try the two parts separately, producing no change. If you try MapAt[Simplify,expr1,{{2,3}}] it thinks you mean MapAt[Simplify,expr1,{2,3}] and complains that there is no part {2,3}. Of course, it can always be done by extracting, Simplifying, and replacing, but I am looking for an easy one liner. Any suggestions? Thanks- Martin McClain, Chem Dept, Wayne State University, Detroit