How do I write an index in a summation?

[fbardo]

I need to expand this summation, but the coefficients a_n are taken as constant, instead of a_1, a_2, ....a_n

I cannot define a_x, because they are unknown, and I need the symbolic answer, not a numerical one.

Also, how do I get the factors of x^1 first? (it starts win x^20)

PD: Can I write latex in this forum?

[RichardJ]

Please post the worksheet (click on "Use advanced editor" at the top right of the edit dialog).

[lwestbrook · Accepted solution]

Hi Forista,

The reason the "n" is being taken as constant is because you are using literal subscripts (Ctrl + -), rather than vector subscripts ([). If you use vector subscripts for a_1 and a_n, you will get the result you need. However, when you try to collect x, the result is so large that Mathcad won't display the result (though it can still be used in calculations). But you can use the simplify or factor keywords to get an actual displayed answer, but it may not take the form you want it to be in (for example, "simplify" will leave all of the (x-1) terms factored rather than multiplying them out. The expansion of these terms is what is causing "collect,x" to give a result too large to display).

[fbardo]

You are right. It works when I use"[" for subindex, and reduce the number of summations to low numbers (like n:0 to 3).

Sadly, It makes Matchcad useless. My summation actually have infinite terms, I put 10 in hope to be able to pick the pattern, and using only 3 or 4 terms is no better than doing it by hand.

I suppose that I need a different software. Do you know something capable of doing symbolic manipulation of series?

[RichardJ]

Mathcad can do symbolic manipulation of series. In general, your expression will not simplify to a finite solution. For summations to N it is the product of an Nth degree polynomial in x and an Nth degree polynomial in (x-1), which is another Nth degree polynomial. As N tends to infinity you have a polynomial with an infinite number of terms. There is no domain for x where that will converge to a finite solution unless specific constraints are imposed on the coefficients, a[n.