Question: How to write a nested loops as follows

 

I think we will write nested for loops or nested sequences. But I can' t achieve.

Could you help me?

EDITED AND EXTENDED:

In fact, the original question is as follows:

and

where it is a standard inner product on L^2 and u(x,t) is a function. For example; u(x,t) =x*t etc.

(The screenshots is taken from a book)

I want to write a code for finding Matrix U whose elements are u_ij.

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