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Hi, i need to write a sum of serie :

with b_k= (k+1)^2-(k)^2 and u^(m-k) is the numerical solution approximation u(x,t_m-k) for k=1,...m-1 and m=1,....,20

for m=1:20

syms k u

b(k)=(k+1)^2-k^2;

F=symsum((b(k)-b(k+1))*u(:,m-k),k,1,m-1)

end

F

but i get error

Necessary I need u(:,m-k)

Please help me

Walter Roberson
on 16 Dec 2020

It is never possible to use a symbolic variable as an index in MATLAB.

You need to create a definite list of values and sum() the list. Or loop adding on to a total.

Note by the way that and so b(k) - b(k+1) = (2*k+1) - (2*(k+1)+1) = -2

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