creating a diagonalmatrix with diag and add a value to it

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hi,
I'm trying to create a diagonal matrix, with two minor diagonals and one major diagonal. I already created the two minor, but I don't know how to create the main one.
Code:
%diagonalmatrix
D = diag((-1)*k_i,-1) + diag(k_i+k_(i+1)) + diag((-1)*k_i, 1);
diag((-1)*k_i,-1) and diag((-1)*k_i, 1) already work.
How to change diag(k_i+k_(i+1)) ?
Must I create a new matrix with the addition already included for the major diagonal? How to?
Thanks!
I included the concept in the attachment.

Respuesta aceptada

Ameer Hamza
Ameer Hamza el 30 de Abr. de 2020
Try this. It shows an example with symbolic variables to show that it creates the correct matrix. You can try it with k as a numeric vector
k = sym('k', [1, 8]);
A = diag(k(2:end-1),1) + diag(k(2:end-1),-1) + diag(k(1:end-1)+k(2:end));
Result
>> A
A =
[ k1 + k2, k2, 0, 0, 0, 0, 0]
[ k2, k2 + k3, k3, 0, 0, 0, 0]
[ 0, k3, k3 + k4, k4, 0, 0, 0]
[ 0, 0, k4, k4 + k5, k5, 0, 0]
[ 0, 0, 0, k5, k5 + k6, k6, 0]
[ 0, 0, 0, 0, k6, k6 + k7, k7]
[ 0, 0, 0, 0, 0, k7, k7 + k8]

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