RE: Help! Sin[n Pi] (n Integer)

*To*: mathgroup at smc.vnet.net*Subject*: [mg9015] RE: [mg8942] Help! Sin[n Pi] (n Integer)*From*: Ersek_Ted%PAX1A at mr.nawcad.navy.mil*Date*: Wed, 8 Oct 1997 00:05:12 -0400*Sender*: owner-wri-mathgroup at wolfram.com

Greg Anderson wrote: ---------- |Does anyone know if there's a way to specify that a variable is an |integer? | | Sin[n Pi] or Cos[(2n+1)/2 Pi] come | up constantly. I can dispose of them with replacement rules, but I was | wondering if there's a way to "tag" n as an integer so that Mma will | automatically kill the Sin and Cos terms later in the derivations. | The following will do what you want: In[1]:= Unprotect[Sin, Cos]; Sin[n_?IntegerQ * Pi]:=0 Cos[n_?OddQ/2 * Pi]:=0 Cos[(m_?EvenQ * n_?IntegerQ + 1)/2 Pi]:=0 Protect[Sin, Cos]; In[4]:= IntegerQ[n]^=True; OddQ[j]^=True; In[5]:= {Sin[n Pi], Cos[(2 n + 1)/2 Pi], Cos[ j / 2 * Pi], Cos[a /2 * Pi]} Out[5]= { 0, 0, 0, Cos[a Pi/2] } Notice: Cos[a/2 * Pi] doesn't simplify because nothing is known about (a). I hope WRI will include the rules above and hundreds of other in a later version. Ted Ersek ersek_ted%pax1a at mr.nawcad.navy.mil