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MathGroup Archive 2002

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Re: Unexpected brainmalfunction [was: Unexpected result with RSolve?]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg33671] Re: [mg33661] Unexpected brainmalfunction [was: Unexpected result with RSolve?]
  • From: Fernando Rinaldi <fernando.rinaldi at e-technik.uni-ulm.de>
  • Date: Sat, 6 Apr 2002 00:48:45 -0500 (EST)
  • References: <a8g3e2$b3i$1@smc.vnet.net> <200204050040.TAA16630@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

wouter.van.den.broeck at vub.ac.be wrote:

> Thanks to all who pointed out the flaw in my deductive thinking ;)
>
> wouter
>
> On Wed, 3 Apr 2002 23:32:18 +0000 (UTC),
> wouter.van.den.broeck at vub.ac.be wrote:
>
> >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
> >
> >
> >

Mathematica is pretty right.....sure! you wrote:
RSolve[{a[n + 1] == n a[n], a[0] == 1}, a[n], n]

so for n=0 you start with:

a[1]=0 a[0]; so a[1]=0
a[2]=1 a[1]    and so a[2]=0
going on a[n]=0


probably you wanted to write this:

RSolve[{a[n ] == (n - 1) a[n-1], a[1] == 1}, a[n], n]
for getting
{{a[n] -> (-1+n)!}}

everything is working

bye



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