Re: Unexpected result with RSolve?

*To*: mathgroup at smc.vnet.net*Subject*: [mg33652] Re: Unexpected result with RSolve?*From*: adam.smith at hillsdale.edu (Adam Smith)*Date*: Thu, 4 Apr 2002 19:40:22 -0500 (EST)*References*: <a8g3e2$b3i$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Your trouble is a simple mistake in doing the transformation. For the example from the Help: RSolve[{a[n] == n a[n-1], a[0] == 1}, a[n], n] You want to replace n with n+1. This gives: RSolve[{a[n + 1] == n+1 a[n], a[0] == 1}, a[n], n] The "n+1 a[n]" vs. "n a[n]" makes a big difference. With "n=0" yours would say a[1] = 0*a[0] = 0*1 = 0. Then all the rest equal n*0. Here is the mathematica output for the correct form: In[3]:= RSolve[{a[n + 1] == (n+1) a[n], a[0] == 1}, a[n], n] Out[3]= {{a[n]-> n!}} As you see it gives the expected results. wouter.van.den.broeck at vub.ac.be wrote in message news:<a8g3e2$b3i$1 at smc.vnet.net>... > Hey, > > There's probably a sound solution, but i'm struggling to grab it: > > RSolve[{a[n + 1] == n a[n], a[0] == 1}, a[n], n] > > returns: > {{a[n] -> 0}} > > where, i believe it 'should' return > {{a[n] -> (n-1)!}} > > Anyone care to give me some directions as to why this 'unexpected' > result? > > (note: I started of with the standard textbook example: > RSolve[{a[n] == n a[n-1], a[0] == 1}, a[n], n] > which returns > {{a[n] -> n!}} > so there's no need to correct me in this direction, I just explored a > bit, and wonder why i'm getting that result with the variation above) > > TIA > > wouter vdb