# How to code block lanczos algorithm?

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Omar B. on 29 Jun 2021
Edited: Omar B. on 2 Jul 2021
Thanks ....
Omar B. on 30 Jun 2021
Any help would be much appreciated.

Steven Lord on 29 Jun 2021
function [T]=bsymLanczos2(A,v,m)
m=13;
n=100;
r = -5 + (5+5).*rand(n);
A = triu(r) + triu(r,1)';
v=rand(n,2); % 2*2 block vector
You ask the user to pass A, v, and m into your function then promptly throw the user-specified values in the garbage and use other values. If you're creating these to test that your function works correctly move them out of the bsymLanczos2 function and pass the variables created by running these lines into bsymLanczos2 as input arguments.
I'm also assuming there are some lines of code beyond what you've posted, as the code as written will only return an all zero matrix. All the code that calculates the alpha, delta, and V arrays has no impact on what bsymLanczos2 returns.
Since I'm guessing this is homework, to check that your code works as you expect see if your textbook has any examples worked out step by step. If it does run your code on the data from one or more of those examples using the debugging tools in MATLAB to step through the code line by line. Check that at each step you get the same answer as in the textbook example and if you don't that you understand why not.
Omar B. on 29 Jun 2021
This is not a homework. I just asked how to initialize a block vectoer because I wrote V(:,:,1) and was not sure about this. Also how to write detla and alpha as block values. I did not post the whole code I need first to find all delta and aplha and then get the matrix T.