How to solve the "You can't use 'k' as an index because it's already defined" issue on Desmos Function Grapher?

Question

My input is:

And I am using the Desmos function grapher which has an issue and the issue is that it alerts me on the third function:

You can't use 'k' as an index because it's already defined.

But 'k' suppose to be local iterative variable defined inside the sum expression only and I expect that 'k' can be redefined, reused and reassigned also somewhere else.

How do I tell it that 'k' is local or to redefine, reuse and reassign it? Is this possible to do that?

Answer 1

When Desmos looks at your third expression, it sees something like (ignoring a lot):

Mathematically speaking, this would be bad notation, so Desmos throws an error. The developers put a lot of thought into balancing expectations of math teachers/students and programmers/computer scientists (c.f. Logs and Infinities), and I think they made the right call here.

You can make use of the total() function to get the result you want: