sum() ind subs()

Posted on 2008-01-20 18:36 By Dirk (

20)

Categories:

Why does Maple not substitute the value for y[1] in the result?

subs(m=3,y[1]=2,sum(r[i]*y[i],i=1..m));%;<br />

I assume that the values were substituted in sum() but subs() does not execute sum(). Accordingly there is at the timepoint of substitution no y[1] in sum()?

Thanks
Dirk

One way

Comment on 2008-01-20 19:34 from jakubi ( Maple Rating 4

923)

This is a way that works, though I do not like much:

eval(sum(r[i]*y[i],i=1..m),[m=3,sum=add,y[1]=2]);

sum and subs

Comment on 2008-01-20 19:54 from Robert Israel ( Maple Rating 4

1460)

The short answer to "why" is that sum(r[i]*y[i], i=1..m) does not actually contain y[1]. See for yourself:

> sum(r[i]*y[i], i=1..m);

sum(r[i]*y[i], i=1..m)

No y[1] there! One thing you could do is this:

> eval(eval(sum(r[i]*y[i],i=1..m),m=3),y[1]=2);

2*r[1]+r[2]*y[2]+r[3]*y[3]

Note: I use eval rather than subs, to have the result evaluated after substitution.

Disregard this post

Comment on 2008-01-21 04:44 from John Fredsted ( Maple Rating 4

566)

Please, disregard this post. The previous content was misconceived.

Yes, but you have insert

Comment on 2008-01-21 04:46 from Dirk (

20)

Yes, but you have insert i=1..3 instead of i=1..m in your equation above. In this case you would not need the m=3 in subs().

Quite correct

Comment on 2008-01-21 04:51 from John Fredsted ( Maple Rating 4

566)

You are quite right, which is why I withdraw my post. I apologize.

I did not see your response,

Comment on 2008-01-21 05:08 from Dirk (

20)

I did not see your response, as I sent my answer. Sorry for that.

I reckoned that

Comment on 2008-01-21 05:15 from John Fredsted ( Maple Rating 4

566)

I reckoned that. I hope that you have received a useful answer from the others, because these postings of mine surely are of no help :-).

It does appear to be the evaluation order.

Comment on 2008-01-21 10:52 from jaytreiman (

41)

Here are two ways of trying to do this. The first does not work,
the sum is not evaluated.

subs(y[1]=2,subs(m=3,sum(r[i]*y[i],i=1..m)));

The next works, with a forced evaluation.

subs(y[1]=2,eval(subs(m=3,sum(r[i]*y[i],i=1..m))));

If you look at the help on sum in Maple 11 you will find
the following statement.

"Maple returns the call unevaluated if it cannot find a closed form."

Since you do not have a closed form, Maple does not evaluate the sum
and there is no "y[1]."

sum() ind subs()

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