       Re: Better recursive method?

• To: mathgroup at smc.vnet.net
• Subject: [mg72818] Re: Better recursive method?
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Sun, 21 Jan 2007 05:21:19 -0500 (EST)
• Organization: Uni Leipzig
• References: <eosh0f\$9bs\$1@smc.vnet.net>

```Hi,

and transform the recursion into and iteration doesn not help ?
and using
pi[0,_] := 0;
pi[1,_] := 1;
pi[i_, theta_] := pi[i, theta] =(2n-1)/(n-1) Cos[theta] pi[i-1, theta]
- n/(n-1) pi[i-2,theta]

would be the first step and
pi[n_, theta_] :=
Module[{p1,p2},
p1=0;
p2=1;
Do[
p1=p2*(2i-1)/(i-1) Cos[theta] -  i/(i-1) p1;
tmp=p2;
p2=p1;
p1=tmp,{i,2,n}]
p2
]
the second one ... BTW why do  you use "i" in the patteren and "n"
in the recursion ??

Regards
Jens

> Hallo:
>
> I have following recursive functions:
> pi(i, theta) := (2n-1)/(n-1) cos(theta) pi(i-1, theta) - n/(n-1)
> pi(i-2,theta)
> tau(i,theta) := n pi(i, theta) - (n+1) pi(i-1,theta)
>
> with intial values:
> pi(0,theta) := 0
> pi(1,theta) := 1
>
> These should calculate the following function:
> S1(theta) := sum(i=1-nstop) (2i+1)/(i(i+1)) (a(i) pi(i,theta) - b(i)
> tau(i,theta)) (where nstop=40)
>
> This should be pretty easy to calculate for Mathematica. I have written
> it the following way:
> pi[0,theta] := 0;
> pi[1,theta] := 1;
> pi[i_, theta_] := pi[i, theta] =(2n-1)/(n-1) Cos[theta] pi[i-1, theta]
> - n/(n-1) pi[i-2,theta]
> tau[i_,theta_] := n pi[i, theta] - (n+1) pi[i-1,theta]
> S1Temp[i_,theta_] := (2i+1)/(i(i+1)) (a[i] pi[i,theta] - b[i]
> tau[i,theta])
> S1(theta) := Sum[ S1Temp[i,theta], {i, 1, nstop}]
>
> Well, it works fine, but it takes enormous time. I have introduced
> recursive function to save time for calculation of Legendre Polynomials
> of which the function 'pi' and 'tau' consist of. But the same function
> if I write with 2 loops each for 'theta' and 'i' in IDL, it calculates
> in fraction of a second. Even the simple calculation of 'pi[40,pi/4]'
> took more than 350 seconds.
>
> What could be the problem in my logic. Is there any other better way to
> write recursive functions in Mathematica?
>
> Any help will be appreciated!
>
> Regards,
> nandan
>

```

• Prev by Date: Re: normalize a table
• Next by Date: RE: how to quickly read a >10MB big file
• Previous by thread: Re: Better recursive method?
• Next by thread: Re: Better recursive method?